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/stop-server.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+# Stop the brainstorm server and clean up
+# Usage: stop-server.sh
+
+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"
+  echo '{"status": "stopped"}'
+else
+  echo '{"status": "not_running"}'
+fi