fix: release the title-generation thread after the title turn

[shoedog]

Problem

After the first turn of a new session, TitleGenerator.generateAndPersist starts an ephemeral thread, runs one turn on it to generate a title, and renames the main thread. Nothing releases that thread: closeSession unsubscribes only the session's own thread id, and it is the only caller of threadUnsubscribe.

Codex unloads a thread only when it has no subscribers and is idle (thread_unload_delay_secs, 60 s by default). So the title thread stays loaded until codex-acp exits, together with every MCP server process Codex started for it from the user's config. A client that opens and closes many sessions over one codex-acp process accumulates one set of those processes per session (#628).

Change

• TitleGenerator.generateAndPersist unsubscribes the ephemeral thread in a finally around the title turn, so it is released whether the turn succeeds or fails.
• The unsubscribe runs before the rename, not after the echo wait, so the thread is not held for the extra 5 s.
• A failed unsubscribe is ignored. Title generation is best-effort, and the title is still written.

Tests

• TitleGenerator.test.ts: the ephemeral thread is unsubscribed after the title turn and when the title turn fails (both fail without the change); the title is still written when the unsubscribe fails; no unsubscribe when thread/start fails.
• session-fork-prompt.test.ts asserted that no thread/unsubscribe is sent at all. It now asserts that none is sent for the fork or its source, which is what the test is about.
• npm run typecheck, npm run build and npm test pass: 121 files and 1728 tests pass, 7 files and 36 tests are skipped.
• Real Codex 0.160.1, with the repro script from Title-generation thread is never unsubscribed, so its MCP server processes outlive session/close #628 against a build of this branch: the title thread's MCP server process exits about 60 s after the title turn, and none is left after both sessions are closed. On main one is left until codex-acp exits.

Fixes #628

🤖 Generated with Claude Code

https://claude.ai/code/session_017izEynabPeSUBT573AtvaZ