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/useChatScrollManager.ts

@@ -457,7 +457,48 @@ export const useChatScrollManager = ({
         const container = scrollRef.current;
         if (!container || typeof ResizeObserver === 'undefined') return;
 
+        let lastScrollHeight = container.scrollHeight;
+        let lastClientHeight = container.clientHeight;
+
         const observer = new ResizeObserver(() => {
+            const nextScrollHeight = container.scrollHeight;
+            const nextClientHeight = container.clientHeight;
+            const scrollHeightChanged = nextScrollHeight !== lastScrollHeight;
+            const clientHeightChanged = nextClientHeight !== lastClientHeight;
+
+            if (clientHeightChanged) {
+                const previousDistanceFromBottom = Math.max(
+                    0,
+                    lastScrollHeight - lastScrollTopRef.current - lastClientHeight,
+                );
+
+                if (isPinnedRef.current) {
+                    const targetScrollTop = Math.max(
+                        0,
+                        nextScrollHeight - nextClientHeight - previousDistanceFromBottom,
+                    );
+
+                    if (Math.abs(container.scrollTop - targetScrollTop) > 0.5) {
+                        markProgrammaticScroll();
+                        container.scrollTop = targetScrollTop;
+                        lastScrollTopRef.current = targetScrollTop;
+                    }
+
+                    lastScrollHeight = nextScrollHeight;
+                    lastClientHeight = nextClientHeight;
+                    updateScrollButtonVisibility();
+                    return;
+                }
+            }
+
+            lastScrollHeight = nextScrollHeight;
+            lastClientHeight = nextClientHeight;
+
+            if (clientHeightChanged && !scrollHeightChanged) {
+                updateScrollButtonVisibility();
+                return;
+            }
+
             schedulePinnedStateAndIndicators();
         });
 
@@ -474,7 +515,7 @@ export const useChatScrollManager = ({
             observer.disconnect();
             childObserver.disconnect();
         };
-    }, [schedulePinnedStateAndIndicators]);
+    }, [schedulePinnedStateAndIndicators, updateScrollButtonVisibility]);
 
     React.useEffect(() => {
         if (typeof window === 'undefined') {