Bridge sessions can idle-exit instead of only completing

What

• The idle-exit helper (triggered by CLAUDE_CODE_EXIT_AFTER_STOP_DELAY) now accepts a callback that runs right before the process actually exits, with errors from it swallowed.
• Bridge-spawned sessions can now be given an idleExitAfterMs value; when greater than 0, the child session gets CLAUDE_CODE_EXIT_AFTER_STOP_DELAY set, and on clean exit its reported status is "idle" instead of "completed".
• New scaffolding computes whether a spawned bridge session should be tagged rc-child, gated behind the tengu_bridge_child_auto_mode flag, and passes it to session spawn along with new projectThreadChild and idleExitAfterMs parameters — though the other two triggering conditions are currently hardcoded to always false.

Why This lays groundwork for bridge sessions to shut themselves down after a period of inactivity rather than running indefinitely or being reported as fully completed, though some of the related triggering logic isn't fully wired up yet.