feat: display agent on-behalf-of user in UI

[michaelva]

Summary

• Add GitHub-style composite avatar for agent tokens showing the on-behalf-of user
• Display side-by-side avatars: agent icon + user/API key avatar overlay
• Show "Agent on behalf of [User]" in popover with full user details
• Add useFetchApiKeyInfo hook for fetching API key information
• Parse extended agent identifier format: agent:accountId:projectId:timestamp:user:userId
• Backward compatible with legacy agent:agent:... format

UI Changes

• AgentAvatar component: New component with composite avatar design
• UserInfo parser: Extended to handle new agent identifier format with on-behalf-of info
• Popover: Displays user name, email, avatar when agent acts on behalf of user
• API key support: Shows API key name and ID when agent acts on behalf of API key

Test Plan

• Agent with on-behalf-of user displays user avatar overlay
• Agent with on-behalf-of API key displays API badge
• Legacy format agent:agent:... parses correctly without crash
• Agent without on-behalf-of displays as generic agent
• Popover shows correct user information

Related

Part of agent token permission fixes - UI component for Part 1 (identifier format and display)

🤖 Generated with Claude Code
Co-Authored-By: Claude Sonnet 4.5 noreply@anthropic.com