Session auto-archive should respect pending work

Problem: Sessions auto-archive on session-end regardless of whether there's outstanding work. A session with an open PR and a pending manual infra step (e.g. a Terraform/deploy action the human still has to take) archives silently, stranding actionable state.

Requests:

1. Don't auto-archive a session that has open/incomplete work (e.g. an open PR it created, or — if the app can read it — unresolved items), or require an explicit confirm.
2. Optionally: expose a pre-close/SessionEnd hook signal that can defer archive (today SessionEnd is purely non-blocking), so teams can gate archive on their own "clean exit" criteria.

Why it matters: The durable handoff record (a committed progress file) mitigates this, but auto-archiving with open work still risks the user overlooking an action that's still theirs to take.