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.

examples/README.md

@@ -38,3 +38,9 @@ Run with: `pnpm run demo:acp` (needs `DEEPSEEK_API_KEY`); `pnpm run demo:code-mo
 The ACP server with **session modes** composed ([`@deepseek-ai/dsh-mode`](../packages/mode/mode)) — the editor's mode picker switches the session into plan mode, the model works under the read-only allowlist, and it leaves through the user-reviewed `exit_plan_mode` tool (the review arrives as an elicitation form).
 
 Run with: `pnpm run demo:plan-acp` (needs `DEEPSEEK_API_KEY`). See [plan-acp-agent/README.md](plan-acp-agent/README.md).
+
+## sandbox-acp-agent
+
+The coding agent with its bash executor swapped for the sandbox stack ([`@deepseek-ai/dsh-sandbox-local`](../packages/sandbox/sandbox-local) + [`@deepseek-ai/dsh-bash-sandbox`](../packages/bash/bash-sandbox) — the one-entry executor swap the `ctx.bash` capability seam exists for), served over ACP with [`@deepseek-ai/dsh-user-approval`](../packages/ui/user-approval) mounted — the first composition where the approval loop is LIVE: a sandbox denial escalated by the model becomes a `session/request_permission` prompt in the editor, and "Allow once" runs exactly that command under the wider mode.
+
+Run with: `pnpm run demo:sandbox-acp` (needs `DEEPSEEK_API_KEY`; bwrap, a Landlock-enforcing kernel, or macOS for confined runs). See [sandbox-acp-agent/README.md](sandbox-acp-agent/README.md).