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/voice/VoiceProvider.tsx

@@ -1,5 +1,11 @@
 import React from 'react';
 import { useVoiceContext } from '@/hooks/useVoiceContext';
+import { useConfigStore } from '@/stores/useConfigStore';
+
+const VoiceContextBridge = React.memo(function VoiceContextBridge() {
+    useVoiceContext();
+    return null;
+});
 
 /**
  * Provider component that initializes voice context sync.
@@ -13,8 +19,12 @@ import { useVoiceContext } from '@/hooks/useVoiceContext';
  * ```
  */
 export function VoiceProvider({ children }: { children: React.ReactNode }) {
-    // Activate session-to-voice sync
-    useVoiceContext();
-    
-    return <>{children}</>;
+    const voiceModeEnabled = useConfigStore((state) => state.voiceModeEnabled);
+
+    return (
+        <>
+            {voiceModeEnabled ? <VoiceContextBridge /> : null}
+            {children}
+        </>
+    );
 }