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.

examples/sandbox-acp-agent/README.md

@@ -0,0 +1,16 @@
+# 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 the **Agent Client Protocol**, plus [`@deepseek-ai/dsh-approval`](../../packages/approval/approval/) — which makes this the first composition where the approval loop is LIVE end to end: bash runs under `read-only`, a denial comes back as the structured marker, the model retries once with `sandbox_permissions` + `justification`, the ACP bridge's answerer turns that ask into a `session/request_permission` prompt in your editor, and "Allow once" runs exactly that command under the wider mode ([sandbox RFC § Escalation](../../docs/rfc/implemented/feature/2026-07-06-sandbox.md)).
+
+```sh
+pnpm run demo:sandbox-acp   # needs DEEPSEEK_API_KEY; drive it from Zed or any ACP client
+```
+
+Zed setup is the same as [acp-agent](../acp-agent/README.md) with this example's command; only the leaf `cordis.yml` differs (the sandbox stack + the approval entry in place of the local bash executor and the extra tool stacks).
+
+- **Every approval is one-shot** (`Allow once` / `Reject` — no `allow_always`: the harness has no grant storage yet), and a dismissed prompt or a rejected ask fails closed with its own error text; so does every ask when no editor is attached to answer.
+- **Two session config options are live** ([sandbox RFC § Per-session mode switching](../../docs/rfc/implemented/feature/2026-07-06-sandbox.md)): a capable client shows `Sandbox` (`read-only`/`workspace-write`/`danger-full-access`) and `Approvals` (`ask`/`never`) selectors per session — a switch is one log-only event on that session's log and execution follows it; the sandbox mode is deliberately NOT stated in the prompt or narrated (the model learns the boundary from the denial marker — behavior, not belief), while an approval switch to `never` is stated and narrated; a resumed session reports its overrides back on `session/load`.
+- **The write boundary is config-fixed**: an escalated `workspace-write` run may write under the launch directory (`workspaceRoot: process.cwd()`) plus the platform temp area — a per-session root is config-phase future work in the [sandbox RFC](../../docs/rfc/implemented/feature/2026-07-06-sandbox.md).
+- **No usable runner fails closed per command** (structured `SANDBOX_UNAVAILABLE`), and the filesystem tools stay unloaded for the same reason as `sandbox-agent`: they would bypass the bash sandbox.
+
+Tests: `tests/escalation.e2e.ts` — keyless, it boots the real `cordis.yml` through the Loader as an ACP subprocess, proves the whole tree (sandbox executor + approval service + bridge) initializes and opens a session, and drives the config options end to end (both advertised with composition currents, switches honored and echoed as complete state, out-of-vocabulary values rejected); with a key and a usable runner, a scripted ACP client plays the human — the real model gets denied, escalates, the client answers `allow-once`, and the retried write must land on disk. `tests/acp.snapshot.ts` (the [shared snapshot kit](../../packages/support/acp-snapshot/) over this composition's `cordis.snapshot.yml` replay overlay) pins four scenarios as committed wire bytes: the keyless config-option exchange, the recorded `mode-switching` arc (the suite's pinned header — both switches, their prompt-section deltas, one "changed by the user" notice per knob, and a confined write landing under the switched mode), and both recorded escalation branches (`session/request_permission` answered allow-once / reject-once). Replay re-executes every recorded bash call under the host's real runner — Seatbelt works out of the box on macOS; on Linux install bubblewrap (or build the Landlock launcher) first, exactly what ci.yml's snapshot lane does. No fixture carries a real denial: denial stderr is backend dialect and would pin a fixture to its recording platform (the rationale comment atop the suite file).