feat(app): add sliding sidebar navigation for mobile

- Homepage now shows "New Agent" form, agent list moves to sidebar
- Hamburger menu replaces back arrow on all screens
- Mobile: full-screen sidebar overlay with swipe-to-close gesture
- Desktop: fixed 320px sidebar, toggle via hamburger
- Smooth bezier-eased animations (220ms)
- Sidebar closes on agent select (mobile only)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/app/src/components/agent-list.tsx

@@ -12,9 +12,10 @@ interface AgentListProps {
   isRefreshing?: boolean;
   onRefresh?: () => void;
   selectedAgentId?: string;
+  onAgentSelect?: () => void;
 }
 
-export function AgentList({ agents, isRefreshing = false, onRefresh, selectedAgentId }: AgentListProps) {
+export function AgentList({ agents, isRefreshing = false, onRefresh, selectedAgentId, onAgentSelect }: AgentListProps) {
   const { theme } = useUnistyles();
   const pathname = usePathname();
   const [actionAgent, setActionAgent] = useState<AggregatedAgent | null>(null);
@@ -65,8 +66,9 @@ export function AgentList({ agents, isRefreshing = false, onRefresh, selectedAge
           agentId,
         },
       });
+      onAgentSelect?.();
     },
-    [isActionSheetVisible, pathname]
+    [isActionSheetVisible, pathname, onAgentSelect]
   );
 
   const handleAgentLongPress = useCallback((agent: AggregatedAgent) => {