Thread lifecycle: pinned threads

[Tryanks]

Summary

Add pinned threads: a user-ordered section at the top of the sidebar.

Spec

• SessionMeta.pinned_at and a user order key within the pinned section.
• Pinning un-settles the thread (reason user) and clears any snooze. Settling unpins.
• Pinning does not exempt a thread from auto-settle; the user can choose keep-active for that.
• Sidebar: pinned section first, manually reorderable; the rest of the order is unchanged.
• Unpin shows the shared undo toast (restores the previous position).
• Protocol: PinSession { session_id, after: Option<String> }, UnpinSession.

Depends on #526.

Reference

T3 Code: pinnedAt / pinOrderKey in packages/contracts/src/orchestration.ts, pin rules in decider.ts.