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/stop-server.sh

@@ -1,14 +1,21 @@
 #!/bin/bash
-# Stop the brainstorm server and clean up
-# Usage: stop-server.sh
+# Stop the brainstorm server and clean up session directory
+# Usage: stop-server.sh <screen_dir>
+
+SCREEN_DIR="$1"
+
+if [[ -z "$SCREEN_DIR" ]]; then
+  echo '{"error": "Usage: stop-server.sh <screen_dir>"}'
+  exit 1
+fi
 
-SCREEN_DIR="${BRAINSTORM_SCREEN_DIR:-/tmp/brainstorm}"
 PID_FILE="${SCREEN_DIR}/.server.pid"
 
 if [[ -f "$PID_FILE" ]]; then
   pid=$(cat "$PID_FILE")
   kill "$pid" 2>/dev/null
-  rm -f "$PID_FILE"
+  # Clean up session directory
+  rm -rf "$SCREEN_DIR"
   echo '{"status": "stopped"}'
 else
   echo '{"status": "not_running"}'