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.

scripts/doc-budgets.manifest.json

@@ -1,11 +1,11 @@
 {
   "AGENTS.md": 1802,
   "docs/AGENTS.md": 1315,
-  "docs/architecture.md": 1642,
+  "docs/architecture.md": 1750,
   "docs/cordis-primer.md": 550,
   "docs/defensive-patterns.md": 550,
   "docs/testing.md": 800,
-  "examples/AGENTS.md": 653,
+  "examples/AGENTS.md": 705,
   "packages/AGENTS.md": 450,
   "packages/README.md": 710
 }