Implement A2A Agent Card discovery per ADR-047 and issue #867:
- AgentCard Pydantic model (agent_card.py) with nested models for
skills, capabilities, extensions, security schemes, and interfaces.
- Factory function build_agent_card() enumerates supported operations
from the facade and maps them to A2A skills (plan-lifecycle,
registry-crud, context-mgmt, entity-sync, namespace-mgmt,
health-diagnostics).
- Version negotiation: supportedVersions and version fields from
A2aVersion constants.
- A2A conformance validation (validate_agent_card_conformance) checks
required fields, version support, and structural integrity.
- Updated ASGI app to build the Agent Card from the facade model
instead of a raw dict, with conformance validation at startup.
- Behave BDD tests: 34 scenarios covering model construction, field
validation, conformance checks, serialization, endpoint responses,
and skill enumeration from facade operations.
- Robot Framework integration tests: 6 test cases for build, conformance,
endpoint, serialization, skill enumeration, and version info.
- Updated A2A package exports and CHANGELOG.
ISSUES CLOSED: #867
