feat: drag-and-drop tab reordering in agent manager (#517)

* tmp

* feat: drag-and-drop tab reordering in agent-manager

Extract tab ordering logic into tab-order.ts with reconcileOrder,
applyTabOrder, firstOrderedTitle. Extract SortableTab and
ConstrainDragYAxis into sortable-tab.tsx. Persist tab order via
WorktreeStateManager to .kilocode/agent-manager.json. Add unit tests.

* cleanup: remove duplicate tests, unused code, and stale re-export

- Delete reorder-tabs.test.ts (merged into tab-order.test.ts)
- Remove reconcileOrder (duplicate of applyTabOrder)
- Remove unused session variable in removeSession
- Remove stale reorderTabs re-export from navigate.ts

* fix: use applyTabOrder for local tab recovery, remove unused pending prop

- Local tab order recovery now uses applyTabOrder (consistent with worktree
  handling), gracefully appending new sessions instead of dropping the order
- Remove unused pending prop from SortableTab component