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/UnifiedControlsDrawer.tsx

@@ -43,21 +43,22 @@ export const UnifiedControlsDrawer: React.FC<UnifiedControlsDrawerProps> = ({
     onOpenModel,
     onOpenEffort,
 }) => {
-    const {
-        providers,
-        currentProviderId,
-        currentModelId,
-        currentVariant,
-        setProvider,
-        setModel,
-        setCurrentVariant,
-        getCurrentModelVariants,
-        getModelMetadata,
-    } = useConfigStore();
-    const { addRecentModel, addRecentEffort, recentEfforts } = useUIStore();
+    const providers = useConfigStore((state) => state.providers);
+    const currentProviderId = useConfigStore((state) => state.currentProviderId);
+    const currentModelId = useConfigStore((state) => state.currentModelId);
+    const currentVariant = useConfigStore((state) => state.currentVariant);
+    const setProvider = useConfigStore((state) => state.setProvider);
+    const setModel = useConfigStore((state) => state.setModel);
+    const setCurrentVariant = useConfigStore((state) => state.setCurrentVariant);
+    const getCurrentModelVariants = useConfigStore((state) => state.getCurrentModelVariants);
+    const getModelMetadata = useConfigStore((state) => state.getModelMetadata);
+    const addRecentModel = useUIStore((state) => state.addRecentModel);
+    const addRecentEffort = useUIStore((state) => state.addRecentEffort);
+    const recentEfforts = useUIStore((state) => state.recentEfforts);
     const { recentModelsList } = useModelLists();
     const currentSessionId = useSessionUIStore((s) => s.currentSessionId);
-    const { saveAgentModelForSession, saveAgentModelVariantForSession } = useSelectionStore();
+    const saveAgentModelForSession = useSelectionStore((state) => state.saveAgentModelForSession);
+    const saveAgentModelVariantForSession = useSelectionStore((state) => state.saveAgentModelVariantForSession);
     const sessionAgentName = useContextStore((state) =>
         currentSessionId ? state.getSessionAgentSelection(currentSessionId) : null
     );