Thread lifecycle: pinned threads and manual arrangement (B2) #528

Summary

Pinned threads and manual arrangement, following T3 Code's current code. Phase B2 of #535. Depends on #526.

Spec

• SessionMeta.pinned_at and pin_order: Option<String>; active_order: Option<String> for manual arrangement of the active list. Order keys are fractional base-26 strings (lexicographic, arbitrary extension, no trailing a), generated by the ordering code in core; the host stores them. Normal moves write only the moved thread's key; corrupt or reversed bounds spread fresh keys while preserving hidden rows' positions.
• Pinning a settled thread promotes it to Active (no unsettled_at stamp). Unpin does not clear Active. Settling unpins and clears the active order. Pinning does not stamp unsettled_at.
• Pinned threads are exempt from automatic settlement, both inactivity and PR routes. (Upstream's user doc says otherwise; its code and test exempt. Code wins.) Manual settle still works and unpins.
• Sidebar: pinned section first, sorted by key then newest-created for keyless rows; drag within pinned or active to reorder, drag between sections to pin / unpin / settle / un-settle, with the action shown while dragging; phone: Arrange threads with handles, plus Move up / Move down in the thread menu. New threads appear above the arranged active rows. Activity never reorders.
• Unpin shows the shared undo toast and restores the former key.
• Protocol: PinSession {session_id, order_key?}, UnpinSession, ReorderPinned {order_key}, ReorderActive {order_key}.

Reference

T3 Code 83a82a46b: Orchestrator.ts pin/unpin/reorder (:2822–2867), ThreadSettlementService.ts:144 (pin blocker), ThreadSettlementService.test.ts:106–126, packages/client-runtime/src/state/threadSort.ts, packages/contracts/src/orchestrationV2.ts:2690.