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/sliding-sidebar.tsx

@@ -11,7 +11,7 @@ import { Gesture, GestureDetector } from "react-native-gesture-handler";
 import { StyleSheet, UnistylesRuntime, useUnistyles } from "react-native-unistyles";
 import { Plus, Settings } from "lucide-react-native";
 import { router } from "expo-router";
-import { useSidebarStore } from "@/stores/sidebar-store";
+import { usePanelStore } from "@/stores/panel-store";
 import { AgentList } from "./agent-list";
 import { useAggregatedAgents } from "@/hooks/use-aggregated-agents";
 import { useSidebarAnimation } from "@/contexts/sidebar-animation-context";
@@ -26,7 +26,15 @@ interface SlidingSidebarProps {
 export function SlidingSidebar({ selectedAgentId }: SlidingSidebarProps) {
   const { theme } = useUnistyles();
   const insets = useSafeAreaInsets();
-  const { isOpen, close } = useSidebarStore();
+  const isMobile =
+    UnistylesRuntime.breakpoint === "xs" || UnistylesRuntime.breakpoint === "sm";
+  const mobileView = usePanelStore((state) => state.mobileView);
+  const desktopAgentListOpen = usePanelStore((state) => state.desktop.agentListOpen);
+  const closeToAgent = usePanelStore((state) => state.closeToAgent);
+
+  // Derive isOpen from the unified panel state
+  const isOpen = isMobile ? mobileView === "agent-list" : desktopAgentListOpen;
+
   const { agents, isRevalidating, refreshAll } = useAggregatedAgents();
   const {
     translateX,
@@ -52,9 +60,6 @@ export function SlidingSidebar({ selectedAgentId }: SlidingSidebarProps) {
     }
   }, [isRevalidating, isManualRefresh]);
 
-  const isMobile =
-    UnistylesRuntime.breakpoint === "xs" || UnistylesRuntime.breakpoint === "sm";
-
   const sortedAgents = useMemo(() => {
     return [...agents].sort((a, b) => {
       if (a.requiresAttention && !b.requiresAttention) return -1;
@@ -70,15 +75,15 @@ export function SlidingSidebar({ selectedAgentId }: SlidingSidebarProps) {
   const hasMore = agents.length > SIDEBAR_AGENT_LIMIT;
 
   const handleClose = useCallback(() => {
-    close();
-  }, [close]);
+    closeToAgent();
+  }, [closeToAgent]);
 
 
   // Mobile: close sidebar and navigate
   const handleCreateAgentMobile = useCallback(() => {
-    close();
+    closeToAgent();
     router.push("/");
-  }, [close]);
+  }, [closeToAgent]);
 
   // Desktop: just navigate, don't close
   const handleCreateAgentDesktop = useCallback(() => {
@@ -87,9 +92,9 @@ export function SlidingSidebar({ selectedAgentId }: SlidingSidebarProps) {
 
   // Mobile: close sidebar and navigate
   const handleSettingsMobile = useCallback(() => {
-    close();
+    closeToAgent();
     router.push("/settings");
-  }, [close]);
+  }, [closeToAgent]);
 
   // Desktop: just navigate, don't close
   const handleSettingsDesktop = useCallback(() => {
@@ -101,17 +106,17 @@ export function SlidingSidebar({ selectedAgentId }: SlidingSidebarProps) {
   const handleAgentSelectMobile = useCallback(() => {
     translateX.value = -windowWidth;
     backdropOpacity.value = 0;
-    close();
-  }, [close, translateX, backdropOpacity, windowWidth]);
+    closeToAgent();
+  }, [closeToAgent, translateX, backdropOpacity, windowWidth]);
 
   const handleViewMore = useCallback(() => {
     if (isMobile) {
       translateX.value = -windowWidth;
       backdropOpacity.value = 0;
     }
-    close();
+    closeToAgent();
     router.push("/agents");
-  }, [backdropOpacity, close, isMobile, translateX, windowWidth]);
+  }, [backdropOpacity, closeToAgent, isMobile, translateX, windowWidth]);
 
   // Close gesture (swipe left to close when sidebar is open)
   const closeGesture = Gesture.Pan()