fix review findings: correct teardown framing (dispose, not cancel+whenIdle)

Codex's second pass caught that the prior doc fix swapped one wrong primitive
for another: framing teardown as cancel()+whenIdle() (or awaiting
agent.whenIdle() on disposal) is still wrong. whenIdle() only OBSERVES
quiescence; cancel() only stops queued/in-flight work. Neither unregisters the
agent or detaches the session. Real teardown is AgentHandle.dispose(), whose
disposer does `stop(); await agent.done` — stop the loop, await its exit, and
unregister (packages/core/agent-loop/src/index.ts:271). Copying the old framing
would reintroduce the orphaned-agent/session leak the AgentHandle seam exists
to prevent.

- docs/architecture.md: whenIdle() is a non-owner quiescence-observation hook,
  explicitly NOT teardown; teardown is `await AgentHandle.dispose()`.
- docs/cookbook/extension-cookbook.md (prose + the ts comment): tear agents
  down via AgentHandle.dispose(), not agent.whenIdle().
- docs/rfc/proposed/feature/2026-06-14-acp-agent-client-protocol.md: the
  lifecycle/disposal paragraph routes teardown through the handle's dispose().

docs/cookbook/extension-cookbook.md

@@ -56,7 +56,7 @@ 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.cancel()`. 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 (handle disposal aborts in-flight work then `await`s `agent.whenIdle()`), not just request it.
+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.cancel()`. 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 tear each agent down through its `AgentHandle.dispose()` (which stops the loop, `await`s its exit, and unregisters), not just `cancel()` — disposal must *reach* quiescence, not merely request it.
 
 `packages/ui/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.
 
@@ -77,7 +77,7 @@ export function apply(ctx: Context) {
     }
   })
   // Inbound "prompt": create/resume an agent and feed it; settle on turn end.
-  // Disposal awaits quiescence: handle disposal aborts, then await agent.whenIdle().
+  // Teardown reaches quiescence via AgentHandle.dispose() (stop + await exit).
 }
 ```