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/stores/sidebar-store.ts

@@ -0,0 +1,30 @@
+import { create } from "zustand";
+import { persist, createJSONStorage } from "zustand/middleware";
+import AsyncStorage from "@react-native-async-storage/async-storage";
+
+interface SidebarState {
+  isOpen: boolean;
+  toggle: () => void;
+  open: () => void;
+  close: () => void;
+}
+
+export const useSidebarStore = create<SidebarState>()(
+  persist(
+    (set) => ({
+      isOpen: false,
+      toggle: () => set((state) => ({ isOpen: !state.isOpen })),
+      open: () => set({ isOpen: true }),
+      close: () => set({ isOpen: false }),
+    }),
+    {
+      name: "sidebar-state",
+      storage: createJSONStorage(() => AsyncStorage),
+      partialize: (state) => ({ isOpen: state.isOpen }),
+    }
+  )
+);
+
+export function useSidebar() {
+  return useSidebarStore();
+}