fix(subagents): report manually quit child sessions

[w-winter]

Fixes a lifecycle gap where a Pi-backed child subagent could be manually quit while the parent kept treating it as still running. Graceful child shutdown now uses the existing .exit sidecar protocol, so the watcher receives a terminal exit record and can clear parent-side state instead of relying on shell/screen sentinel behavior.

I believe the issue originates in the initial .exit sidecar design: subagent_done and caller_ping wrote terminal sidecars, but quitting the child session directly didn't. b4b0287 made that direct quit observable via session_shutdown("quit"); this PR wires that event into the same terminal-exit contract.

The implementation logic: child sessions classify their own exits as done, ping, error, or quit, and the parent renders manual quit as "closed by user" rather than task completion or failure. The child-side sidecar claim is process-local and uses exclusive create semantics, so the first successful writer wins and later shutdown bookkeeping can't reclassify an already-claimed done / ping / error exit as a stale quit after the parent consumes the sidecar.

Summary of changes:

• child extension: add an idempotent .exit sidecar writer with atomic first-writer-wins semantics
• child extension: write done for normal auto-exit and preserve error, ping, and explicit subagent_done sidecars
• child extension: write quit on unclaimed session_shutdown("quit") so manual child exits notify the parent
• parent watcher: decode { type: "quit" } as a terminal quit reason
• parent result flow: thread exitReason through subagent completion, resume handling, result details, and rendering
• UI: present manual child shutdown as "closed by user" instead of completed or failed
• tests: cover sidecar writes, non-overwrite behavior, stale-quit race protection, quit decoding, resume fallback, and result rendering

Verification:

• unit: npm test / node --test test/test.ts → 138/138 passing (with a local-only patch for a confirmed-preexisting planner fixture mismatch)
• integration: full cmux integration suite passed with PI_TEST_MODEL=openai-codex/gpt-5.4-mini (I'm without Claude in Pi currently) → 15/15 passing
• manual: spawned an auto-exit: false worker, quit the child with /quit, and confirmed the parent emitted Sub-agent "ManualQuitCheck" closed by user, not completed/failed
• manual: resumed a previously completed subagent session after its sidecar had been consumed; confirmed it accepted the follow-up and did not immediately emit a stale closed by user result

Screenshots:

• Manually exiting subagent sessions without interrupting subagent turns:

• Manually exiting a subagent session and, in the process of doing so, interrupting a subagent turn: