perf(sidebar): index session ownership and narrow live subscriptions

Replace repeated project-by-session directory matching across sidebar hooks with a shared ownership index that resolves each unique directory once and exposes direct project and folder-scope buckets.

Gate destructive folder reconciliation on authoritative session data and topology readiness, preserve last-known worktrees after discovery failures, and retain nested-project, VS Code, active/archive dedupe, and Windows drive-root semantics.

Narrow cross-directory subscriptions to session and status slices so streaming deltas no longer trigger global aggregation. Reuse a cached session ID index for permission lineage checks instead of rebuilding it on every session switch.

On the reported 15-project, 67-worktree, 14,561-session shape, ownership indexing averages 3.81 ms versus roughly 450 ms for the cache-only hotfix.

Validation: 28 targeted tests, UI type-check, UI lint, and dead-code analysis.

packages/ui/src/sync/DOCUMENTATION.md

@@ -80,6 +80,10 @@ Current consumers:
 - `Header.tsx`
 - agent/session activity surfaces using `useGlobalSessionStatus()` / `useAllSessionStatuses()`
 
+Cross-directory selectors subscribe to the narrow child-store field they aggregate. Session aggregation listens to `state.session`; per-session status listens only to that session's `state.session_status` entry. Unrelated streaming events such as `message.part.delta` must not trigger global session/status scans.
+
+Imperative cross-directory session lookups use the cached ID index from `getAllSyncSessionMap()`. The index is rebuilt only when a child store's `state.session` reference changes; permission lineage checks must reuse it instead of rebuilding a full session map per call.
+
 ### Mutation responsibility
 
 `useGlobalSessionsStore` is not maintained by SSE directly. It is kept correct by: