Thread lifecycle: snooze

[Tryanks]

Summary

Add snooze: hide a thread from the active list until a chosen time, following T3 Code.

Spec

• SessionMeta.snoozed_until, snoozed_at. Snooze is an overlay on active, not a fourth destination.
• Presets: 1 hour, 3 hours, this evening at 18:00 (only when more than an hour away), tomorrow at 09:00, next Monday at 09:00, custom.
• The wake time must be in the future. Pending approval, pending user input or a queued turn blocks snoozing; a running session does not.
• Waking: the host schedules a wake at snoozed_until (reuse the scheduled-wake mechanism). A user message clears the snooze.
• Raise a hand: a snoozed thread reappears in the active list, without clearing the snooze, on an approval or input request, a fresh error, or a completion after the snooze started.
• Settling clears snooze; pinning clears snooze.
• Sidebar: a collapsed "Snoozed" section between active and settled, showing the wake time.
• Undo toast, shared with settle.
• Protocol: SnoozeSessions { session_ids, until }, UnsnoozeSessions.

Depends on #526 (shared state model, sweep and undo toast).

Reference

T3 Code: migration 034, decider.ts snooze rules, threadSettled.ts presets and raise-hand rules, #4311.