Track container-only tokens and tab focus for cmd+j ranking

Previously ranked by whether any container-only matches existed (boolean);
now counts tokens matching only containers for finer-grained ranking. Tab
focus recency is now tracked explicitly so recent refocuses rank above
stale worktree activity. Preserves worktree grouping by input order while
applying focused-group MRU within each block.

src/shared/workspace-session-schema.ts

@@ -147,6 +147,8 @@ const tabSchema = z.object({
   color: z.string().nullable(),
   sortOrder: z.number(),
   createdAt: z.number(),
+  // Why: corrupt optional recency must not discard the whole persisted tab.
+  lastFocusedAt: z.number().finite().nonnegative().optional().catch(undefined),
   isPreview: z.boolean().optional(),
   isPinned: z.boolean().optional(),
   // Why: persist the per-tab native-chat view mode so 'chat' survives reload /