Move brainstorm server metadata to .meta/ subdirectory

Metadata files (.server-info, .events, .server.pid, .server.log,
.server-stopped) were stored in the same directory served over HTTP,
making them accessible via the /files/ route. They now live in a .meta/
subdirectory that is not web-accessible.

Also fixes a stale test assertion ("Waiting for Claude" → "Waiting for
the agent").

Reported-By: 吉田仁

skills/brainstorming/scripts/start-server.sh

@@ -83,11 +83,12 @@ else
   SCREEN_DIR="/tmp/brainstorm-${SESSION_ID}"
 fi
 
-PID_FILE="${SCREEN_DIR}/.server.pid"
-LOG_FILE="${SCREEN_DIR}/.server.log"
+META_DIR="${SCREEN_DIR}/.meta"
+PID_FILE="${META_DIR}/.server.pid"
+LOG_FILE="${META_DIR}/.server.log"
 
-# Create fresh session directory
-mkdir -p "$SCREEN_DIR"
+# Create fresh session directory and metadata subdirectory
+mkdir -p "$SCREEN_DIR" "$META_DIR"
 
 # Kill any existing server
 if [[ -f "$PID_FILE" ]]; then