feat(v2): track unregistered senders + setup improvements

- Add unregistered_senders table to capture dropped message origins
  (one row per sender, upserted with message_count and last_seen)
- Add inbound DM logging to chat-sdk-bridge for debugging
- Add vercel CLI to base container image
- Install vercel-cli and frontend-engineer container skills
- Default requiresTrigger to false in register step

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

container/Dockerfile

@@ -31,7 +31,7 @@ ENV AGENT_BROWSER_EXECUTABLE_PATH=/usr/bin/chromium
 ENV PLAYWRIGHT_CHROMIUM_EXECUTABLE_PATH=/usr/bin/chromium
 
 # Install agent-browser and claude-code globally
-RUN npm install -g agent-browser @anthropic-ai/claude-code
+RUN npm install -g agent-browser @anthropic-ai/claude-code vercel
 
 # Create app directory
 WORKDIR /app