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/hooks/useMessageTTS.ts

@@ -24,18 +24,16 @@ export interface UseMessageTTSReturn {
 export function useMessageTTS(): UseMessageTTSReturn {
     const [isPlaying, setIsPlaying] = useState(false);
     
-    const {
-        voiceProvider,
-        speechRate,
-        speechPitch,
-        speechVolume,
-        sayVoice,
-        browserVoice,
-        openaiVoice,
-        summarizeMessageTTS,
-        summarizeCharacterThreshold,
-        showMessageTTSButtons,
-    } = useConfigStore();
+    const voiceProvider = useConfigStore((state) => state.voiceProvider);
+    const speechRate = useConfigStore((state) => state.speechRate);
+    const speechPitch = useConfigStore((state) => state.speechPitch);
+    const speechVolume = useConfigStore((state) => state.speechVolume);
+    const sayVoice = useConfigStore((state) => state.sayVoice);
+    const browserVoice = useConfigStore((state) => state.browserVoice);
+    const openaiVoice = useConfigStore((state) => state.openaiVoice);
+    const summarizeMessageTTS = useConfigStore((state) => state.summarizeMessageTTS);
+    const summarizeCharacterThreshold = useConfigStore((state) => state.summarizeCharacterThreshold);
+    const showMessageTTSButtons = useConfigStore((state) => state.showMessageTTSButtons);
 
     const shouldCheckOpenAIAvailability = showMessageTTSButtons && voiceProvider === 'openai';
     const shouldCheckSayAvailability = showMessageTTSButtons && voiceProvider === 'say';