fix(electrobun): PathBrowser uses unguarded files.browse RPC (dirs only)

- files.browse: new RPC handler — unguarded directory listing, returns
  only directories (no file content access). Used by PathBrowser wizard.
- PathBrowser: uses files.browse instead of files.list (was blocked by
  guardPath "access denied: path outside allowed project directories")
- Home dir resolved via files.homeDir RPC (not process.env.HOME)

Note: GTK native dialog title/theme/sort controlled by Electrobun's
native wrapper — canChooseFiles:false should set SELECT_FOLDER action
but may need upstream fix for correct title.

ui-electrobun/src/shared/pty-rpc-schema.ts

@@ -146,6 +146,11 @@ export type PtyRPCRequests = {
       error?: string;
     };
   };
+  /** Unguarded directory listing for PathBrowser (dirs only, no file content) */
+  "files.browse": {
+    params: { path: string };
+    response: { entries: { name: string; type: 'dir'; size: number }[]; error?: string };
+  };
   /** Native folder picker dialog */
   "files.pickDirectory": {
     params: { startingFolder?: string };