Consolidate Blender agent into Agent UI

[kovtcharov]

Part of #759

The BlenderAgent (src/gaia/agents/blender/) provides 3D scene automation via Blender. Currently accessed via gaia blender CLI.

• Ensure BlenderAgent is selectable in Agent UI's agent picker
• Handle Blender-specific output (3D renders, scene previews) in Agent UI
• Verify MCP integration works through Agent UI