- Create ChatAgentSelector with Popover+Command dropdown
- Show active agent icon, name, and ChevronDown indicator
- 'Select agent' placeholder when no agent selected
- 'No agents configured' empty state via CommandEmpty
- Agent list shows icon, name, and role label per item
- Selection calls onAgentChange and PATCHes conversation via chatApi
- Role-specific colors from agentRoleColors applied to agent icons
- Loading state shows Skeleton placeholder
- Create chat.ts API client with updateConversation supporting agentId
- Create shared types/chat.ts with ChatMessage, ChatConversation types
- Create ChatCodeBlock prerequisite from phase-21 base
- TypeScript compiles clean
- ChatMarkdownMessage: renders markdown with rehype-highlight syntax highlighting
- ChatCodeBlock: pre override with language label and copy button (1500ms success state)
- Uses ExtraProps from react-markdown for correct TypeScript types
- All tests pass, TypeScript compiles without errors
