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.base.json

@@ -45,14 +45,17 @@
         "./packages/bash/*/src",
         "./packages/code-runtime/*/src",
         "./packages/fs/*/src",
+        "./packages/skill/*/src",
         "./packages/compact/*/src",
         "./packages/guard/*/src",
         "./packages/mode/*/src",
         "./packages/subagent/*/src",
+        "./packages/workflow/*/src",
         "./packages/web/*/src",
         "./packages/timeout/*/src",
         "./packages/todo/*/src",
         "./packages/cordis/*/src",
+        "./packages/sandbox/*/src",
         "./packages/hooks/*/src",
         "./packages/session-persistence/*/src",
         "./packages/ui/*/src",