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).

AGENTS.md

@@ -33,10 +33,15 @@ packages/    Harness packages, all named @deepseek-ai/dsh-<name>:
   bash/           abstract bash executor seam (ctx.bash) — interface only
   bash-local/     local-subprocess BashExecutor implementation
   tool-bash/      model-facing bash/bash_output/bash_kill tool schemas
+  acp/            Agent Client Protocol bridge: drive the agent from an ACP
+                  editor (Zed) over JSON-RPC stdio
 examples/    Runnable demos (not workspaces). echo-agent = mock model + echo
              tool + stdio UI + JSONL persistence, wired via cordis.yml.
              coding-agent = the real thing: DeepSeek V4 + bash tools
              (yarn demo:coding, needs DEEPSEEK_API_KEY).
+             acp-agent = the coding agent exposed as an ACP server over
+             JSON-RPC stdio (yarn demo:acp, needs DEEPSEEK_API_KEY).
+             base.yml = shared provider/tool core both real demos include.
 docs/        architecture.md — the design doc. adr/ — decision records (the
              why behind vendoring, event-sourcing, the schema DSL, …).
              rfc/ — proposals for substantial future work.
@@ -72,6 +77,9 @@ yarn demo:echo      # run examples/echo-agent (no API key; type "echo hi" to
                     # see a tool call) — the mock skeleton
 yarn demo:coding    # run examples/coding-agent — the real agent (needs
                     # DEEPSEEK_API_KEY; give it a coding task)
+yarn demo:acp       # run examples/acp-agent — the coding agent as an ACP
+                    # server over JSON-RPC stdio (needs DEEPSEEK_API_KEY;
+                    # drive it from Zed or another ACP client)
 ```
 
 ## Secrets / .env