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/ModelControls.tsx

@@ -293,25 +293,23 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
     onMobilePanelSelection,
     onAgentPanelSelection,
 }) => {
-    const {
-        providers,
-        currentProviderId,
-        currentModelId,
-        currentVariant,
-        currentAgentName,
-        settingsDefaultVariant,
-        settingsDefaultAgent,
-        setProvider,
-        setSelectedProvider,
-        setModel,
-        setCurrentVariant,
-        getCurrentModelVariants,
-        setAgent,
-        getCurrentProvider,
-        getModelMetadata,
-        getCurrentAgent,
-        getVisibleAgents,
-    } = useConfigStore();
+    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 currentAgentName = useConfigStore((state) => state.currentAgentName);
+    const settingsDefaultVariant = useConfigStore((state) => state.settingsDefaultVariant);
+    const settingsDefaultAgent = useConfigStore((state) => state.settingsDefaultAgent);
+    const setProvider = useConfigStore((state) => state.setProvider);
+    const setSelectedProvider = useConfigStore((state) => state.setSelectedProvider);
+    const setModel = useConfigStore((state) => state.setModel);
+    const setCurrentVariant = useConfigStore((state) => state.setCurrentVariant);
+    const getCurrentModelVariants = useConfigStore((state) => state.getCurrentModelVariants);
+    const setAgent = useConfigStore((state) => state.setAgent);
+    const getCurrentProvider = useConfigStore((state) => state.getCurrentProvider);
+    const getModelMetadata = useConfigStore((state) => state.getModelMetadata);
+    const getCurrentAgent = useConfigStore((state) => state.getCurrentAgent);
+    const getVisibleAgents = useConfigStore((state) => state.getVisibleAgents);
 
     // Use visible agents (excludes hidden internal agents)
     const agents = getVisibleAgents();
@@ -321,15 +319,13 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
     const getDirectoryForSession = useSessionUIStore((s) => s.getDirectoryForSession);
     const sync = useSync();
 
-    const {
-        getSessionModelSelection,
-        saveSessionModelSelection,
-        saveSessionAgentSelection,
-        saveAgentModelForSession,
-        getAgentModelForSession,
-        saveAgentModelVariantForSession,
-        getAgentModelVariantForSession,
-    } = useSelectionStore();
+    const getSessionModelSelection = useSelectionStore((state) => state.getSessionModelSelection);
+    const saveSessionModelSelection = useSelectionStore((state) => state.saveSessionModelSelection);
+    const saveSessionAgentSelection = useSelectionStore((state) => state.saveSessionAgentSelection);
+    const saveAgentModelForSession = useSelectionStore((state) => state.saveAgentModelForSession);
+    const getAgentModelForSession = useSelectionStore((state) => state.getAgentModelForSession);
+    const saveAgentModelVariantForSession = useSelectionStore((state) => state.saveAgentModelVariantForSession);
+    const getAgentModelVariantForSession = useSelectionStore((state) => state.getAgentModelVariantForSession);
 
     const contextHydrated = useContextStore((state) => state.hasHydrated);
 
@@ -355,19 +351,17 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
         ? (sessionSavedAgentName || stickySessionAgentName || currentAgentName)
         : currentAgentName;
 
-    const {
-        toggleFavoriteModel,
-        isFavoriteModel,
-        collapsedModelProviders,
-        toggleModelProviderCollapsed,
-        addRecentModel,
-        addRecentAgent,
-        addRecentEffort,
-        isModelSelectorOpen,
-        setModelSelectorOpen,
-        setSettingsDialogOpen,
-        setSettingsPage,
-    } = useUIStore();
+    const toggleFavoriteModel = useUIStore((state) => state.toggleFavoriteModel);
+    const isFavoriteModel = useUIStore((state) => state.isFavoriteModel);
+    const collapsedModelProviders = useUIStore((state) => state.collapsedModelProviders);
+    const toggleModelProviderCollapsed = useUIStore((state) => state.toggleModelProviderCollapsed);
+    const addRecentModel = useUIStore((state) => state.addRecentModel);
+    const addRecentAgent = useUIStore((state) => state.addRecentAgent);
+    const addRecentEffort = useUIStore((state) => state.addRecentEffort);
+    const isModelSelectorOpen = useUIStore((state) => state.isModelSelectorOpen);
+    const setModelSelectorOpen = useUIStore((state) => state.setModelSelectorOpen);
+    const setSettingsDialogOpen = useUIStore((state) => state.setSettingsDialogOpen);
+    const setSettingsPage = useUIStore((state) => state.setSettingsPage);
     const hiddenModels = useUIStore((state) => state.hiddenModels);
     const collapsedProviderSet = React.useMemo(
         () => new Set(collapsedModelProviders.map((providerId) => providerId.trim()).filter(Boolean)),