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.

tsconfig.json

@@ -31,7 +31,11 @@
     { "path": "./packages/core/system-prompt" },
     { "path": "./packages/core/agent" },
     { "path": "./packages/ui/user-interaction" },
+    { "path": "./packages/ui/user-approval" },
     { "path": "./packages/core/tools" },
+    { "path": "./packages/skill/skill" },
+    { "path": "./packages/skill/skill-local" },
+    { "path": "./packages/skill/tool-skill" },
     { "path": "./packages/ui/tool-ask-user" },
     { "path": "./packages/core/agent-loop" },
     { "path": "./packages/core/agent-core" },
@@ -41,6 +45,9 @@
     { "path": "./packages/llm/llm-deepseek" },
     { "path": "./packages/llm/llm-pi-ai" },
     { "path": "./packages/bash/bash-local" },
+    { "path": "./packages/sandbox/sandbox" },
+    { "path": "./packages/sandbox/sandbox-local" },
+    { "path": "./packages/bash/bash-sandbox" },
     { "path": "./packages/bash/tool-bash" },
     { "path": "./packages/fs/fs" },
     { "path": "./packages/fs/fs-local" },
@@ -66,9 +73,13 @@
     { "path": "./packages/support/subagent-mock" },
     { "path": "./packages/subagent/tool-subagent" },
     { "path": "./packages/subagent/subagent-inprocess" },
+    { "path": "./packages/subagent/subagent-subprocess" },
     { "path": "./packages/subagent/subagent-spawn" },
     { "path": "./packages/subagent/subagent-fork" },
     { "path": "./packages/subagent/subagent-acp" },
+    { "path": "./packages/workflow/workflow" },
+    { "path": "./packages/workflow/workflow-workerthread" },
+    { "path": "./packages/workflow/tool-workflow" },
     { "path": "./packages/todo/tool-todo" },
     { "path": "./packages/mode/mode" },
     { "path": "./packages/guard/repeat-tool-guard" },