As a registry user, I want to discover and manage A2A agent cards through the registry, so that I can find A2A-compatible agents alongside MCP servers and skills

[rdimitrov]

Acceptance Criteria

0. Update the registry types to support A2A
1. New A2A enum value and a2a_agent_card extension table for A2A-specific metadata (agent URL, capabilities, authentication)
2. CRUD API endpoints support the a2a entry type
3. Listing/filtering endpoints return A2A entries
4. Data sources (Git, API, file) can ingest A2A agent card definitions
5. Unit tests cover all new A2A paths