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/useBrowserVoice.ts

@@ -132,7 +132,19 @@ export function useBrowserVoice(): UseBrowserVoiceReturn {
   const sendMessage = useSessionUIStore((s) => s.sendMessage);
   const setPendingInputText = useInputStore((s) => s.setPendingInputText);
   const createSession = useSessionUIStore((s) => s.createSession);
-  const { currentProviderId, currentModelId, currentAgentName, voiceModeEnabled, voiceProvider, speechRate, speechPitch, speechVolume, sayVoice, browserVoice, openaiVoice, summarizeVoiceConversation, summarizeCharacterThreshold } = useConfigStore();
+  const currentProviderId = useConfigStore((state) => state.currentProviderId);
+  const currentModelId = useConfigStore((state) => state.currentModelId);
+  const currentAgentName = useConfigStore((state) => state.currentAgentName);
+  const voiceModeEnabled = useConfigStore((state) => state.voiceModeEnabled);
+  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 summarizeVoiceConversation = useConfigStore((state) => state.summarizeVoiceConversation);
+  const summarizeCharacterThreshold = useConfigStore((state) => state.summarizeCharacterThreshold);
 
   const shouldCheckOpenAIAvailability = voiceModeEnabled && voiceProvider === 'openai';
   const shouldCheckSayAvailability = voiceModeEnabled && voiceProvider === 'say';