fix(session): persist interrupted child sessions

[Sam Schillace (ramparte)]

Summary

• Persist interrupted spawned and resumed child sessions with status: interrupted, preserving the partial transcript and reconstruction metadata for later recovery.
• Centralize child lifecycle teardown so completion-hook registration, cancellation propagation, display nesting, and child cleanup are each released exactly once, even under repeated cancellation or teardown failures.
• Close the pre-save cancellation race by preparing persistence state before execution and recovering when cancellation lands either during execution or during the post-execute transcript read.
• Shield the single finalizer task so subsequent cancellation cannot interrupt persistence or cleanup, while preserving the original cancellation for the caller.
• Ignore .next/ working-session artifacts.

This is the coordinated prerequisite for the Foundation delegate-timeout rollout: Foundation can release the parent promptly at its deadline while this app layer completes interrupted-child persistence and cleanup in the detached task.

Verification

• 12 focused interruption/lifecycle tests passed.
• 142 relevant session-spawner and resumption tests passed.
• 1,313 full-suite tests passed, with the repository's known skips/deselections.
• Ruff clean.
• Pyright clean.