Run automatic devlog recording after exit

Summary

Changed the automatic devlog shutdown hook to hand the session transcript to a detached supervisor, allowing Pi to exit immediately while devlog generation continues in the background. The supervisor records output, enforces a timeout, and removes the temporary handoff directory when finished.

Decisions

• Wait only until the detached supervisor process is spawned, rather than waiting for model and tool execution
• Keep worker reports in the Pi auto-devlog directory and make the detached worker responsible for transcript cleanup

Learnings

• A detached supervisor preserves timeout, reporting, and cleanup behavior without blocking the session shutdown hook