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/agent-tutorial.html

@@ -227,5 +227,6 @@ app.<span class="fn">listen</span>(<span class="num">3000</span>);</code></pre>
   </div>
 </div>
 
+<script src="https://hermes.squaremcp.com/chat-widget.js"></script>
 </body>
 </html>