refactor(app): unified panel state machine for mobile sidebars

Replace separate sidebar stores with a unified panel store that uses a
discriminated union state machine on mobile. This makes it impossible
for both the agent list and file explorer sidebars to be open at the
same time on mobile.

Mobile state machine (mobileView):
- 'agent': Main agent view (no overlay panel)
- 'agent-list': Agent list sidebar (left overlay)
- 'file-explorer': File explorer sidebar (right overlay)

Desktop retains independent boolean toggles since sidebars sit
alongside content rather than overlaying it.

Key changes:
- Created stores/panel-store.ts with unified state
- Deleted stores/sidebar-store.ts and explorer-sidebar-store.ts
- Updated all components to use new panel store
- Animation contexts derive isOpen from unified state

packages/app/src/components/headers/menu-header.tsx

@@ -3,7 +3,7 @@ import { Pressable, Text } from "react-native";
 import { StyleSheet, UnistylesRuntime, useUnistyles } from "react-native-unistyles";
 import { Menu, PanelLeft } from "lucide-react-native";
 import { ScreenHeader } from "./screen-header";
-import { useSidebarStore } from "@/stores/sidebar-store";
+import { usePanelStore } from "@/stores/panel-store";
 
 interface MenuHeaderProps {
   title?: string;
@@ -12,10 +12,13 @@ interface MenuHeaderProps {
 
 export function MenuHeader({ title, rightContent }: MenuHeaderProps) {
   const { theme } = useUnistyles();
-  const { isOpen, toggle } = useSidebarStore();
   const isMobile =
     UnistylesRuntime.breakpoint === "xs" || UnistylesRuntime.breakpoint === "sm";
+  const mobileView = usePanelStore((state) => state.mobileView);
+  const desktopAgentListOpen = usePanelStore((state) => state.desktop.agentListOpen);
+  const toggleAgentList = usePanelStore((state) => state.toggleAgentList);
 
+  const isOpen = isMobile ? mobileView === "agent-list" : desktopAgentListOpen;
   const MenuIcon = isMobile ? Menu : PanelLeft;
   const menuIconColor = !isMobile && isOpen
     ? theme.colors.foreground
@@ -25,7 +28,7 @@ export function MenuHeader({ title, rightContent }: MenuHeaderProps) {
     <ScreenHeader
       left={
         <>
-          <Pressable onPress={toggle} style={styles.menuButton}>
+          <Pressable onPress={toggleAgentList} style={styles.menuButton}>
             <MenuIcon size={16} color={menuIconColor} />
           </Pressable>
           {title && (