Merge branch 'worktree-session-modes-rfc' (master: sandbox stack #169, skills #109, prompt snapshots #254)

The stack rebases onto a moved master through its base branch. Beyond
mechanical unions (both branches' demo scripts, example rows, service
roles, tool lists, acp deps, doc budgets — each side fit alone, the
union needs the higher ceilings), three semantic reconciliations:

- The ACP bridge now carries BOTH per-session surfaces: the sandbox
  stack's config options + approval answerer and this branch's session
  modes; session/new and session/load advertise modes AND configOptions
  side by side.
- The feature matrix supersedes the sandbox stance per the RFC's
  second-lander rule: session/set_mode and current_mode_update flip to
  shipped-by-dsh-mode, config-option rows stay as #169 wrote them, and
  §6 records both landed features under the picker-to-modes /
  knobs-to-config-options division.
- The snapshot pin grammar (#254: one header snapshot + declared deltas
  + a Markdown prompt golden) gains a symmetric declaration for what a
  delta cannot express: expectedHeaderSnapshots — a plan-mode flip
  resorts the canonical tool list, so its widening lands as a second
  full snapshot, now its own Markdown section. The pin-less-class and
  model-turn-only-pin amendments carry over; new fixtures cover the
  extended writer paths, and the plan-acp-agent scenarios re-recorded
  under the merged composition (the app now bundles the skill tool)
  with the suite's refresh mode wired through.

docs/tool-catalog.md

@@ -21,8 +21,10 @@ This table connects model-visible tool names to the plugin package and service s
 | `@deepseek-ai/dsh-tool-bash` | `bash`, `bash_kill`, `bash_output` | `ctx.tools`, `ctx.bash` | `tool/call`, `tool/result`, `context/message via agent.inject() for background completion notices` | - | The bash/bash_output/bash_kill tools are model-facing consumers of the bash executor seam. |
 | `@deepseek-ai/dsh-tool-cordis` | `cordis_inspect`, `cordis_mount`, `cordis_unmount` | `ctx.tools` | `tool/call`, `tool/result`, `live plugin-tree mutations (mount/unmount)` | - | Ships in examples/cordis-agent only (a deliberate opt-in — mounted code gets the real ctx, see docs/rfc/implemented/feature/2026-07-08-self-referential-cordis-toolset.md). Plugins the model mounts may register ADDITIONAL model-visible tools at runtime; the request-header ToolsDelta logs those tool-set changes. |
 | `@deepseek-ai/dsh-tool-fs` | `edit`, `read`, `write` | `ctx.tools`, `ctx.fs`, `ctx.systemPrompt` | `tool/call`, `fs/write-intent or fs/edit-intent for mutations`, `fs/observed after successful file operations`, `tool/result` | - | The read-before-write/edit policy is added by `@deepseek-ai/dsh-fs-policy` (an `fs/*` event-gate plugin, no schema change); a deployment that loads these tools is expected to also load it. The tool schemas above are identical with or without the policy plugin. |
+| `@deepseek-ai/dsh-tool-skill` | `skill` | `ctx.tools`, `ctx.skills` | `tool/call`, `tool/result` | - | - |
 | `@deepseek-ai/dsh-tool-subagent` | `subagent` | `ctx.tools`, `ctx.subagents` | `tool/call`, `tool/result`, `child session events through the chosen provider` | `subagent`, `subagent_fork` | The registered tool name is the load-time `toolName` config (default `subagent`); the schema above is that default. The shipped example agents load this package once per subagent backend, so the model additionally sees `subagent_fork` (bound to the fork backend) with an identical schema — see `examples/coding-agent/cordis.yml` and `examples/acp-agent/cordis.yml`. |
 | `@deepseek-ai/dsh-tool-todo` | `todo_write` | `ctx.tools`, `owning Agent session` | `tool/call`, `todo/write`, `tool/result` | - | todo_write is session-owned state; UIs render the latest todo/write event as a checklist or ACP plan. |
+| `@deepseek-ai/dsh-tool-workflow` | `workflow` | `ctx.tools`, `ctx.workflows`, `ctx.systemPrompt`, `a calling Agent (exec.agent parents the script children)` | `tool/call`, `tool/result` | - | - |
 | `@deepseek-ai/dsh-tool-web` | `web_fetch`, `web_search` | `ctx.tools`, `ctx.web`, `ctx.systemPrompt` | `tool/call`, `tool/result` | - | web_search and web_fetch keep provider selection behind ctx.web so model-visible schemas stay stable across backend swaps. |
 
 ## `@deepseek-ai/dsh-tool-ask-user`
@@ -149,7 +151,7 @@ exit_plan_mode presents the plan for the user's review over the user-interaction
 
 ### `bash`
 
-Execute a bash command (`bash -c`) and return its stdout/stderr. Each call runs in a fresh shell: no state (cwd, variables, functions) persists between calls — pass `workdir` instead of using `cd`. Non-zero exits are reported as `[exit code: N]`. Long output is truncated to its tail; the full output is saved to a file whose path is reported when available. Set `run_in_background: true` for long-running commands: the call returns a task id immediately; poll it with `bash_output` and stop it with `bash_kill`.
+Execute a bash command (`bash -c`) and return its stdout/stderr. Each call runs in a fresh shell: no state (cwd, variables, functions) persists between calls — pass `workdir` instead of using `cd`. Non-zero exits are reported as `[exit code: N]`. Commands may run under a file sandbox; a blocked file operation is reported as `[sandbox: file access denied under <mode> mode]` — a policy denial, not a bug in the command; do not retry another way (a background task reports the same marker via bash_output once it has finished). Long output is truncated to its tail; the full output is saved to a file whose path is reported when available. Set `run_in_background: true` for long-running commands: the call returns a task id immediately; poll it with `bash_output` and stop it with `bash_kill`.
 
 ```json
 {
@@ -395,6 +397,29 @@ Source: [`packages/fs/tool-fs/src/index.ts`](../packages/fs/tool-fs/src/index.ts
 
 The read-before-write/edit policy is added by `@deepseek-ai/dsh-fs-policy` (an `fs/*` event-gate plugin, no schema change); a deployment that loads these tools is expected to also load it. The tool schemas above are identical with or without the policy plugin.
 
+## `@deepseek-ai/dsh-tool-skill`
+
+### `skill`
+
+Load the full instructions for an available skill. Call this with the exact skill name from the session skill catalog before acting on a task that names or clearly matches that skill.
+
+```json
+{
+  "type": "object",
+  "properties": {
+    "name": {
+      "type": "string",
+      "description": "The exact skill name from the available skills list."
+    }
+  },
+  "required": [
+    "name"
+  ]
+}
+```
+
+Source: [`packages/skill/tool-skill/src/index.ts`](../packages/skill/tool-skill/src/index.ts)
+
 ## `@deepseek-ai/dsh-tool-subagent`
 
 ### `subagent`
@@ -472,6 +497,92 @@ Source: [`packages/todo/tool-todo/src/index.ts`](../packages/todo/tool-todo/src/
 
 todo_write is session-owned state; UIs render the latest todo/write event as a checklist or ACP plan.
 
+## `@deepseek-ai/dsh-tool-workflow`
+
+### `workflow`
+
+Run a JavaScript workflow script that orchestrates subagents at scale. Use this for work that fans out across many independent pieces — an audit over many files, a migration, multi-angle research, adversarial verification of findings — where you write the orchestration as a script instead of delegating turn by turn.
+
+The workflow's identity rides the `meta` parameter as JSON: required `name` (short kebab-case) and `description` strings, optional `whenToUse` string and `phases` array (`{title, detail?, model?}`). The `script` parameter is the plain JavaScript body ONLY (NOT TypeScript, and NO `export const meta` statement — meta is a parameter, not code), running with top-level await; end with `return <value>` — the value must be JSON-serializable and is this tool's result.
+
+Script-body hooks:
+- `agent(prompt, opts?): Promise<any>` — run one subagent to completion. Without `opts.schema` it resolves to the child's final text; with `opts.schema` (an object-rooted JSON Schema using ONLY type/properties/required/additionalProperties/items/enum/const — no oneOf/pattern/format/numeric bounds) it resolves to the validated object. Resolves `null` when the child fails (filter with `.filter(Boolean)`). Other opts: `label` (display), `phase` (progress group), `model` (override). Anything else (`effort`/`isolation`/`agentType`) is rejected loudly.
+- `pipeline(items, ...stages): Promise<any[]>` — run each item through the stages independently with NO barrier between stages (prefer this for multi-stage work). Each stage receives `(prev, item, index)`. An ordinary stage throw drops that ITEM to `null` and skips its remaining stages.
+- `parallel(thunks): Promise<any[]>` — run zero-argument functions concurrently and await ALL of them (a barrier; use only when a stage genuinely needs every prior result together). A throwing thunk resolves to `null`.
+- `phase(title)` — start a progress phase; `log(message)` — narrate progress; `args` — the tool call's `args` input, verbatim.
+
+Misused hooks (bad arguments, unknown options, unsupported schemas, tripped caps) throw errors that ALWAYS kill the script — they never dissolve into a per-item `null`.
+
+Constraints: concurrency and total-agent caps apply; no filesystem, network, timers, or Node.js APIs are provided — the agents do the work, the script only coordinates them. The run executes in the foreground: this call returns when the whole script finishes.
+
+```json
+{
+  "type": "object",
+  "properties": {
+    "script": {
+      "type": "string",
+      "description": "The plain-JS workflow script body (top-level await allowed; NO `export const meta` statement; end with `return <json-value>`)."
+    },
+    "meta": {
+      "type": "object",
+      "description": "The workflow identity block (plain JSON — never code).",
+      "properties": {
+        "name": {
+          "type": "string",
+          "description": "Short kebab-case workflow name."
+        },
+        "description": {
+          "type": "string",
+          "description": "One-line description of what the workflow does."
+        },
+        "whenToUse": {
+          "type": "string",
+          "description": "Optional guidance on when this workflow applies."
+        },
+        "phases": {
+          "type": "array",
+          "description": "Optional phase declarations matched by phase() calls.",
+          "items": {
+            "type": "object",
+            "properties": {
+              "title": {
+                "type": "string",
+                "description": "The phase title phase() calls match by exact string."
+              },
+              "detail": {
+                "type": "string",
+                "description": "Optional one-line description of the phase."
+              },
+              "model": {
+                "type": "string",
+                "description": "Optional model override this phase is expected to use."
+              }
+            },
+            "required": [
+              "title"
+            ]
+          }
+        }
+      },
+      "required": [
+        "name",
+        "description"
+      ]
+    },
+    "args": {
+      "type": "object",
+      "description": "Optional JSON input exposed to the script as the `args` global (wrap a bare list as a field, e.g. {\"files\": [...]})."
+    }
+  },
+  "required": [
+    "script",
+    "meta"
+  ]
+}
+```
+
+Source: [`packages/workflow/tool-workflow/src/index.ts`](../packages/workflow/tool-workflow/src/index.ts)
+
 ## `@deepseek-ai/dsh-tool-web`
 
 ### `web_fetch`