codex runtime: stop the Flair task loop on shutdown #781

Found while triaging a CI failure on #759 (Docker Integration, run 42243604181). The failure was in review-trigger-loop.test.ts, whose warnings contained [testbot] Task loop catch-up read unavailable (ConnectionRefused). Those warnings came from the task loop that codex-presence-444.test.ts starts for testbot, still polling in the same Bun process.

runCodexRuntime (packages/cli/src/utils/codex-runtime.ts ~831) starts the Flair task loop and keeps no stop handle. shutdownHandler (~702) stops only the heartbeat before the final beat and exit(0).

What the fix does:

• The runtime keeps the task loop's stop handle. The SIGINT/SIGTERM shutdown handler stops the task loop together with the heartbeat, before the final Presence beat. The startup-failure path stops it too.
• codex-presence-444.test.ts asserts that no catch-up request reaches its fake server after the shutdown handler ran, so no loop outlives the test.