fix: session isolation and blocking wait for visual companion

- Each session gets unique temp directory (/tmp/brainstorm-{pid}-{timestamp})
- Server outputs screen_dir and screen_file in startup JSON
- stop-server.sh takes screen_dir arg and cleans up session directory
- Document blocking TaskOutput pattern: 10-min timeouts, retry up to 3x,
  then prompt user "let me know when you want to continue"

lib/brainstorm-server/index.js

@@ -81,5 +81,11 @@ chokidar.watch(SCREEN_FILE).on('change', () => {
 });
 
 server.listen(PORT, '127.0.0.1', () => {
-  console.log(JSON.stringify({ type: 'server-started', port: PORT, url: `http://localhost:${PORT}` }));
+  console.log(JSON.stringify({
+    type: 'server-started',
+    port: PORT,
+    url: `http://localhost:${PORT}`,
+    screen_dir: SCREEN_DIR,
+    screen_file: SCREEN_FILE
+  }));
 });