feat(example): sandbox-acp-agent — the live composition; RFCs to implemented

The three-entry cordis.yml (dsh-sandbox-local + dsh-bash-sandbox at a
read-only default + dsh-approval) served over ACP: the first live approval
composition. Recorded snapshot scenarios pin the wire end to end —
config-options advertisement, the mode-switching arc as the suite pinned
header (both switches, the prompt-section delta, one changed-by-the-user
notice per knob, a confined write landing under the switched mode), and
both escalation branches over scripted permissionAnswers (a grant runs
confined under workspace-write; a rejection executes nothing and pins the
fail-closed text). The with-key escalation e2e drives a real model +
real runner + the real bridge answerer, world-verified; ci.yml snapshot
lane and e2e.yml install bubblewrap so the confined replays actually
execute.

Both RFCs move to implemented/ (Decision/Consequences form, deferred
phases tracked in their own sections), with every cross-reference flipped.

package.json

@@ -68,6 +68,7 @@
     "demo:code-mode": "node scripts/demo-code-mode.mjs",
     "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: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": {