perf: reduce UI render fanout and scroll jitter

- Cut broad render fanout across the app by replacing shared-store whole-object subscriptions with leaf selectors, memoizing hot chrome boundaries, and isolating disabled global providers from live session/message state. This keeps header controls, composer toolbars, side panels, and other non-hot UI surfaces from repainting on every assistant update or keystroke.

- Rework sidebar session ordering so recent, project groups, and worktree groups derive from one ordering source while avoiding streaming-time thrash. The sidebar now uses a stabilized session snapshot, preserves structural identity for unchanged rows, reads live row status/details per session, and applies a one-shot sort bump on idle->busy instead of continuously resorting during activity.

- Fix chat/input scroll instability by separating viewport-resize handling from message-growth handling, disabling conflicting native scroll anchoring, and stopping textarea autosize from collapsing on every growth keystroke. This removes the multiline typing jiggle during streaming and reduces unnecessary composer rerenders.

- Also gate voice context wiring behind voice-mode enablement and codify the learned render/scroll/order anti-patterns in AGENTS.md so future changes avoid the same classes of regressions.

packages/ui/src/components/session/sidebar/utils.tsx

@@ -133,6 +133,20 @@ export const compareSessionsByPinnedAndTime = (
   return getSessionUpdatedAt(b) - getSessionUpdatedAt(a);
 };
 
+export const compareSessionsByPinnedAndCreated = (
+  a: Session,
+  b: Session,
+  pinnedSessionIds: Set<string>,
+): number => {
+  const aPinned = pinnedSessionIds.has(a.id);
+  const bPinned = pinnedSessionIds.has(b.id);
+  if (aPinned !== bPinned) {
+    return aPinned ? -1 : 1;
+  }
+
+  return getSessionCreatedAt(b) - getSessionCreatedAt(a);
+};
+
 export const dedupeSessionsById = (sessions: Session[]): Session[] => {
   const byId = new Map<string, Session>();
   sessions.forEach((session) => {