ui: Avatar + Badge + Tooltip + @mention autocomplete

- MessageItem: shadcn Avatar (square, brutalist), Badge for AGT/type labels
- Tooltip on timestamps: tap/hover shows full date + seq number
- ComposeBox: typing @ opens mention autocomplete popup
  - Arrow keys to navigate, Enter/Tab to select, Escape to close
  - Shows username, display name, role
  - Fetches user list on mount
- More breathing room in messages (py-3/4)

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

ui/colony/package.json

@@ -18,6 +18,7 @@
     "@tailwindcss/vite": "^4.2.2",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
+    "cmdk": "^1.1.1",
     "lucide-react": "^1.7.0",
     "react": "^19.2.4",
     "react-dom": "^19.2.4",