feat: Slack platform + Claude-powered chat support widget

- Add Slack as customer-facing messaging platform (client, 4 MCP tools, dashboard card)
- Add /api/chat endpoint powered by Claude Haiku with SquareMCP system prompt
- Add embeddable chat-widget.js injected into all 3 sites (docs, app, www)
- Add ANTHROPIC_API_KEY, serve product/ as static files
- Update Platform type to include slack

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/index.html

@@ -90,5 +90,6 @@ document.querySelectorAll('.nav-links a').forEach(a => {
   if (a.href === location.href) a.classList.add('active');
 });
 </script>
+<script src="https://hermes.squaremcp.com/chat-widget.js"></script>
 </body>
 </html>