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).

examples/coding-agent/start.ts

@@ -4,11 +4,16 @@ import Loader from '@cordisjs/plugin-loader'
 
 // Load DEEPSEEK_API_KEY / DEEPSEEK_BASE_URL from a gitignored repo-root .env
 // (Node >= 21.7 native). Absent file is fine — the environment may already
-// carry the variables; cordis.yml reads them via the `!!js` tag.
+// carry the variables; cordis.yml reads them via the `!!js` tag. A
+// present-but-unreadable/malformed .env is a real misconfiguration: surface it
+// rather than silently running with the wrong environment.
 try {
   process.loadEnvFile(new URL('../../.env', import.meta.url).pathname)
-} catch {
-  // no .env — rely on the ambient environment
+} catch (error) {
+  if ((error as NodeJS.ErrnoException | null)?.code !== 'ENOENT') {
+    process.stderr.write(`coding-agent: failed to load .env: ${String(error)}\n`)
+  }
+  // ENOENT (no .env) is fine — rely on the ambient environment.
 }
 
 // Boot a Cordis app from this example's cordis.yml — the same shape as the