perf(chat): make session switching feel instant

Switching sessions ran as one synchronous commit: sidebar highlight, URL,
a full timeline remount with markdown re-parse, and around nine requests,
so nothing changed on screen for 150-250ms after the click.

- ChatContainer swaps the timeline on a deferred copy of the selection, so
  the active row, URL, and tab commit first and the timeline renders behind
  them; selection policy keeps reading the live store value.
- The message fetch starts before the selection is published.
- Sidebar rows stop re-rendering on a project switch: directory-scoped sync
  hooks read the runtime context and a subscribable current-directory source
  instead of the directory-bearing context; the grouping builder reads git
  branches through a ref and section caches key the branches they use;
  descendant ids are keyed by content. Rows per switch went from 73 to 8.
- Markdown skips the async re-render when the settled cached blocks are
  already painted, and mounts synchronously once its lazy module is loaded;
  the module is preloaded at boot.
- A timeline reveal gate holds a freshly opened session at opacity 0 while
  any provisional markdown paint catches up (250ms cap), then fades the whole
  timeline in once, so text, tools, and recap appear together.
- Switch fan-out trimmed: knowledge summary deduped, MCP status refreshed only
  when stale, non-repo directories cached by the git repo check, OpenChamber
  defaults cached briefly, agent memory reused for the same project, goal
  text cached, PWA manifest rebuilt after the switch settles.
- Header tabs snap into the active state and keep the title at the same
  height in both states.
- Prefetch on row press; composer focus moved off the commit.

`bun run profile:switch` records ack/content latency, longest task, and
requests per switch, cold and warm, and compares runs against a baseline.
Measured warm switch: ack 228ms to about 40-60ms, content 228ms to about
100-120ms.

packages/ui/src/components/chat/ChatInput.tsx

@@ -72,8 +72,8 @@ import { useChatSearchDirectory } from '@/hooks/useChatSearchDirectory';
 import { opencodeClient } from '@/lib/opencode/client';
 import { useGitStore, useIsGitRepo } from '@/stores/useGitStore';
 import { useDirectoryStore } from '@/stores/useDirectoryStore';
-import { useSkillsStore } from '@/stores/useSkillsStore';
-import { useCommandsStore } from '@/stores/useCommandsStore';
+import { selectSkillsForDirectory, useSkillsStore } from '@/stores/useSkillsStore';
+import { selectCommandsForDirectory, useCommandsStore } from '@/stores/useCommandsStore';
 import { useRuntimeAPIs } from '@/hooks/useRuntimeAPIs';
 import { useEffectiveDirectory } from '@/hooks/useEffectiveDirectory';
 import { usePermissionStore } from '@/stores/permissionStore';
@@ -602,8 +602,8 @@ const ChatInputComponent: React.FC<ChatInputProps> = ({
 
     // Known slash-invocations (commands + skills + built-ins) used to highlight
     // matching /tokens in the composer, the same way confirmed @files are.
-    const availableCommands = useCommandsStore((s) => s.commands);
-    const availableSkills = useSkillsStore((s) => s.skills);
+    const availableCommands = useCommandsStore((s) => selectCommandsForDirectory(s, currentDirectory));
+    const availableSkills = useSkillsStore((s) => selectSkillsForDirectory(s, currentDirectory));
     const knownSlashNames = React.useMemo(() => {
         const names = new Set<string>([
             'init', 'review', 'undo', 'redo', 'timeline', 'compact', 'btw', 'summary', 'workspace-review', 'plan-feature', 'craft-goal', 'schedule-task', 'catch-up', 'debug', 'weigh', 'explore',
@@ -1140,7 +1140,7 @@ const ChatInputComponent: React.FC<ChatInputProps> = ({
             : [];
 
         const availableSkillNames = new Set(
-            useSkillsStore.getState().skills.map((skill) => skill.name),
+            selectSkillsForDirectory(useSkillsStore.getState(), currentDirectory).map((skill) => skill.name),
         );
 
         const outgoing = buildOutgoingMessage({
@@ -2262,10 +2262,14 @@ const ChatInputComponent: React.FC<ChatInputProps> = ({
     };
 
     React.useEffect(() => {
-
-        if (active && currentSessionId && composerRef.current && !isMobile) {
-            composerRef.current.focus();
-        }
+        if (!active || !currentSessionId || isMobile) return;
+        // Focusing forces layout. Right after a session switch the layout is
+        // dirty from the whole timeline mounting, so the focus call would pay
+        // for that layout inside the commit; a frame later it is nearly free.
+        const frame = window.requestAnimationFrame(() => {
+            composerRef.current?.focus();
+        });
+        return () => window.cancelAnimationFrame(frame);
     }, [active, currentSessionId, isMobile]);
 
     React.useEffect(() => {