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/layout/Sidebar.tsx

@@ -16,7 +16,8 @@ interface SidebarProps {
 }
 
 export const Sidebar: React.FC<SidebarProps> = ({ isOpen, isMobile, children, className }) => {
-    const { sidebarWidth, setSidebarWidth } = useUIStore();
+    const sidebarWidth = useUIStore((state) => state.sidebarWidth);
+    const setSidebarWidth = useUIStore((state) => state.setSidebarWidth);
     const isDesktopApp = React.useMemo(() => isDesktopShell(), []);
     const [isResizing, setIsResizing] = React.useState(false);
     const startXRef = React.useRef(0);