refactor: remove old git-diff and file-explorer screens

- Delete standalone /git-diff and /file-explorer routes
- Update agent menu to open explorer sidebar instead of navigating
- Update file path clicks in chat to open sidebar
- Fix unnecessary re-renders in sidebar panes by using agents.has() instead of agents.get()

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

packages/app/src/components/git-diff-pane.tsx

@@ -218,11 +218,11 @@ export function GitDiffPane({ serverId, agentId }: GitDiffPaneProps) {
     agentId,
   });
 
-  const agent = useSessionStore((state) =>
-    state.sessions[serverId]?.agents?.get(agentId)
+  const agentExists = useSessionStore((state) =>
+    state.sessions[serverId]?.agents?.has(agentId) ?? false
   );
 
-  if (!agent) {
+  if (!agentExists) {
     return (
       <View style={styles.errorContainer}>
         <Text style={styles.errorText}>Agent not found</Text>