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.

package.json

@@ -22,6 +22,7 @@
     "test:e2e": "vitest run --config vitest.e2e.config.ts",
     "test:snapshot": "vitest run --config vitest.snapshot.config.ts",
     "test:snapshot:record": "DSH_SNAPSHOT=record vitest run --config vitest.snapshot.config.ts --update",
+    "test:snapshot:refresh": "DSH_SNAPSHOT=refresh vitest run --config vitest.snapshot.config.ts",
     "check:ci": "tsx scripts/run-gates.ts ci-primary",
     "check:ci:static": "tsx scripts/run-gates.ts ci-static",
     "check:ci:lint": "tsx scripts/run-gates.ts ci-lint",
@@ -69,6 +70,7 @@
     "demo:cordis": "node --expose-internals --import tsx packages/ui/stdio-agent/src/bin.ts examples/cordis-agent/cordis.yml",
     "demo:acp": "node --import tsx packages/ui/acp-agent/src/bin.ts examples/acp-agent/cordis.yml",
     "demo:plan-acp": "node --import tsx packages/ui/acp-agent/src/bin.ts examples/plan-acp-agent/cordis.yml",
+    "demo:sandbox-acp": "node --import tsx packages/ui/acp-agent/src/bin.ts examples/sandbox-acp-agent/cordis.yml",
     "postinstall": "node scripts/install-lefthook.mjs"
   },
   "devDependencies": {