SessionDelete hook

Goal

Notify a user's hook after an explicitly selected session has been deleted.

Contract

• SessionDelete runs after SessionService.removeSession or removeSessions reports that a transcript was removed.
• The hook is fire-and-forget. Its output and failure cannot undo or delay a completed deletion.
• The payload contains the hook runtime's normal hook fields plus deleted_session_id. The hook runtime owns the hook configuration; the deleted session can be inactive and has no live hook runtime.
• The interactive /delete flow and ACP's explicit deleteSession extension method emit the event. Cleanup, rollback, archive, close, and daemon REST batch deletion do not.

Rationale

SessionEnd describes an active conversation lifecycle. Permanent deletion is storage lifecycle work and can target an inactive transcript, so it needs a separate event and identifier. Running it only after success prevents hooks from leaving close-and-delete flows partially completed.

Daemon REST deletion has no Config or HookSystem owner in the process that removes transcripts. Wiring that path would require an explicit workspace-hook execution contract, rather than reconstructing a deleted session's in-memory hooks. It is intentionally out of scope for this change.