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/index.js

@@ -5,7 +5,8 @@ const chokidar = require('chokidar');
 const fs = require('fs');
 const path = require('path');
 
-const PORT = process.env.BRAINSTORM_PORT || 3333;
+// Use provided port or pick a random high port (49152-65535)
+const PORT = process.env.BRAINSTORM_PORT || (49152 + Math.floor(Math.random() * 16383));
 const SCREEN_FILE = process.env.BRAINSTORM_SCREEN || '/tmp/brainstorm/screen.html';
 const SCREEN_DIR = path.dirname(SCREEN_FILE);