- Create ChatMessageIdentityBar with agent icon, name, timestamp, and streaming dot
- Create ChatStreamingCursor with animate-cursor-blink and aria-hidden
- Extend ChatMessage with agentName, agentIcon, agentRole, timestamp, isStreaming props
- Wrap assistant messages in group div for hover actions (Plan 03)
- Create agent-role-colors.ts with 11 distinct role colors (light + dark variants)
- Create ChatMarkdownMessage prerequisite from phase-21 base
- All 4 ChatMessageIdentityBar tests pass
