feat(acp): ACP bridge — drive the coding agent from an editor over JSON-RPC stdio

Implements the RFC 010 MVP: a new `@deepseek-ai/dsh-acp` package bridges the
harness agent to the Agent Client Protocol (JSON-RPC 2.0 over newline-delimited
stdio), so Zed and other ACP editors can drive the coding agent — streaming
render, tool-call display, and resumable sessions via `session/load`.

- packages/acp: AgentSideConnection wiring; initialize/newSession/loadSession/
  prompt/cancel; a total TurnEndReason→StopReason codec; settle-once with a
  fallback chain (agent/turn-end → logged turn/end → idle); single-session
  guard; cwd-must-equal-launch-dir validation; load replays from the persisted
  event log (assistant/chunk→agent_message_chunk, tool/call/result→tool_call*).
- agent: add Agent.whenIdle() quiescence signal to the interface; LoopAgent
  implements it (resolves on the first running→idle/disposed transition). The
  bridge awaits it on disposal so teardown reaches quiescence, not just abort.
- examples: extract the shared provider/tool core into examples/base.yml;
  coding-agent nest-includes it; new examples/acp-agent serves the agent over
  ACP with JSONL persistence and no stdout logger (stdout is the protocol).
- Permission gate deferred (TODO(rfc010-permission-gate)): tools run with the
  executor's full authority; only the Agent→sessionId ownership seam is laid
  down. Cancel is best-effort for a not-yet-started queued turn
  (TODO(rfc010-cancel-prestep)). RFC 010 stays `proposed`.
- Docs: package README + Zed snippet; client-driver cookbook section; root and
  packages layout/commands; RFC 010 implementation-status note.

48 bridge tests + whenIdle coverage; 100% per-file coverage; e2e boots the
example as a subprocess and verifies a written file on disk (key-gated, with a
no-key stdout-purity check).

docs/cookbook/extension-cookbook.md

@@ -53,6 +53,33 @@ export function apply(ctx: Context) {
 }
 ```
 
+## A client-driver plugin (external protocol bridge)
+
+A *client driver* is a UI plugin whose "user" is another program speaking a wire protocol rather than a human at a terminal. It owns the process's stdio (so it must run with **no stdout logger** — every non-protocol byte corrupts the stream), creates/resumes agents on demand through the `dsh-agent` factory seam, translates harness events (`session/event`, `agent/*`) into outbound protocol messages, and translates inbound requests back into `agent.send()` / `agent.abort()`. Two harness-specific contracts make it correct: resolve each request exactly once off a settle signal (the turn can end without its `agent/turn-end` event firing — fall back through the logged `turn/end` record), and on disposal reach quiescence (`await agent.whenIdle()` after `abort()`), not just request it.
+
+`packages/acp` is the worked example: it bridges the agent to the Agent Client Protocol (JSON-RPC over stdio) so Zed and other ACP editors can drive it. See its README for the full method surface and the deferred-permission-gate note.
+
+```ts
+import type { Context } from 'cordis'
+
+export const name = 'my-protocol-bridge'
+export const inject = ['agents', 'sessions', 'sessionPersistence']
+
+export function apply(ctx: Context) {
+  // Stream every logged assistant text/reasoning delta out to the client.
+  ctx.on('session/event', (_session, event) => {
+    if (event.type === 'assistant/chunk') {
+      const chunk = event.data.chunk
+      if (chunk.type === 'text-delta') {
+        // sendToClient({ kind: 'message_chunk', text: chunk.text })
+      }
+    }
+  })
+  // Inbound "prompt": create/resume an agent and feed it; settle on turn end.
+  // Disposal awaits quiescence: agent.abort() then await agent.whenIdle().
+}
+```
+
 ## Runnable wirings
 
-Two complete examples load their plugin trees from `cordis.yml` with HMR: [`examples/echo-agent`](../../examples/echo-agent) (mock model + echo tool — the all-mock skeleton check, `yarn demo:echo`) and [`examples/coding-agent`](../../examples/coding-agent) (DeepSeek V4 + the bash tool suite — the real thing, `yarn demo:coding`).
+Three complete examples load their plugin trees from `cordis.yml` with HMR: [`examples/echo-agent`](../../examples/echo-agent) (mock model + echo tool — the all-mock skeleton check, `yarn demo:echo`), [`examples/coding-agent`](../../examples/coding-agent) (DeepSeek V4 + the bash tool suite — the real thing, `yarn demo:coding`), and [`examples/acp-agent`](../../examples/acp-agent) (the same coding agent exposed as an ACP server over JSON-RPC stdio — the client-driver shape, `yarn demo:acp`). The two real demos share their provider/tool core via [`examples/base.yml`](../../examples/base.yml).