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/chat/ChatContainer.tsx

@@ -104,7 +104,9 @@ export const ChatContainer: React.FC = () => {
     );
 
     // UI store
-    const { isExpandedInput, stickyUserHeader, chatRenderMode } = useUIStore();
+    const isExpandedInput = useUIStore((state) => state.isExpandedInput);
+    const stickyUserHeader = useUIStore((state) => state.stickyUserHeader);
+    const chatRenderMode = useUIStore((state) => state.chatRenderMode);
 
     // Streaming state
     const streamingMessageId = useStreamingStore(
@@ -516,9 +518,7 @@ export const ChatContainer: React.FC = () => {
                         <ScrollShadow
                             className="absolute inset-0 overflow-y-auto overflow-x-hidden z-0 chat-scroll overlay-scrollbar-target"
                             ref={scrollRef}
-                            style={(timelineController.pendingRevealWork || timelineController.isLoadingOlder)
-                                ? { overflowAnchor: 'none' }
-                                : undefined}
+                            style={{ overflowAnchor: 'none' }}
                             observeMutations={false}
                             hideTopShadow={isMobile && stickyUserHeader}
                             data-scroll-shadow="true"