feat: add visual companion for brainstorming skill

Adds browser-based mockup display to replace ASCII art during
brainstorming sessions. Key components:

- Frame template with OS-aware light/dark theming
- CSS helpers for options, cards, mockups, split views
- Server lifecycle scripts (start/stop with random high port)
- Event watcher using tail+grep for feedback loop
- Claude instructions for using the visual companion

The skill now asks users if they want browser mockups and only
runs in Claude Code environments.

lib/brainstorm-server/start-server.sh

@@ -0,0 +1,42 @@
+#!/bin/bash
+# Start the brainstorm server and output connection info
+# Usage: start-server.sh
+#
+# Starts server on a random high port, outputs JSON with URL
+# Server runs in background, PID saved for cleanup
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+SCREEN_DIR="${BRAINSTORM_SCREEN_DIR:-/tmp/brainstorm}"
+SCREEN_FILE="${SCREEN_DIR}/screen.html"
+PID_FILE="${SCREEN_DIR}/.server.pid"
+LOG_FILE="${SCREEN_DIR}/.server.log"
+
+# Ensure screen directory exists
+mkdir -p "$SCREEN_DIR"
+
+# Kill any existing server
+if [[ -f "$PID_FILE" ]]; then
+  old_pid=$(cat "$PID_FILE")
+  kill "$old_pid" 2>/dev/null
+  rm -f "$PID_FILE"
+fi
+
+# Start server, capturing output to log file
+cd "$SCRIPT_DIR"
+node index.js > "$LOG_FILE" 2>&1 &
+SERVER_PID=$!
+echo "$SERVER_PID" > "$PID_FILE"
+
+# Wait for server-started message (check log file)
+for i in {1..50}; do
+  if grep -q "server-started" "$LOG_FILE" 2>/dev/null; then
+    # Extract and output the server-started line
+    grep "server-started" "$LOG_FILE" | head -1
+    exit 0
+  fi
+  sleep 0.1
+done
+
+# Timeout - server didn't start
+echo '{"error": "Server failed to start within 5 seconds"}'
+exit 1