GAIA AI PC Agents Hub Roadmap Updates (#282)

## Summary

Updates to the Agent Hub roadmap documentation including Arena redesign,
roadmap integration, and CUA seed agent.

## Changes

### Agent Arena Redesign
- Renamed "Arena" to "Agent Arena" throughout documentation
- Redesigned arena mockup as side-by-side agent comparison (Chatbot
Arena style)
- Added suggested queries feature - users can select from predefined
queries or write custom

### Roadmap Integration
- Added "AI PC Agents Hub" to main roadmap (`docs/roadmap.mdx`)
- Added Q2B node to mermaid timeline diagram
- Added full Q2 2026 section with CardGroup and capabilities

### Seed Agents
- Added ComputerUseAgent (CUA) to seed agents table
- Added "desktop automation" as competition theme option

## Files Changed

- `docs/roadmap.mdx` - Added Agent Hub to timeline and Q2 section
- `docs/plans/agent-hub.mdx` - Arena rename, CUA seed agent, changelog
- `docs/plans/mockups/arena.html` - Redesigned with suggested queries

## Testing

- Verified all pages render in Mintlify dev server
- Tested mockup interactivity in browser

Author: kovtcharov

docs/docs.json

@@ -305,6 +305,7 @@
           {
             "group": "Plans",
             "pages": [
+              "plans/agent-hub",
               "plans/installer",
               "plans/chat-ui",
               "plans/mcp-client",