[adapters] A2A bidirectional adapter — export agent cards and discovery

[dgenio]

Problem

The A2A adapter only converts A2A agents to SelectableItems (inbound). There is no way to expose a contextweaver-managed agent's capabilities back to A2A peers. This makes contextweaver a consumer-only participant in multi-agent systems.

Proposal

Add outbound A2A conversion:

1. selectable_to_a2a_agent_card(item, endpoint_url) — Convert a SelectableItem to an A2A agent card dict
2. catalog_to_a2a_discovery(catalog, base_url) — Generate A2A discovery response listing all catalog items as agent skills
3. Round-trip support — a2a_agent_to_selectable() → selectable_to_a2a_agent_card() preserves key fields

Acceptance Criteria

• selectable_to_a2a_agent_card() produces valid A2A agent card dicts
• catalog_to_a2a_discovery() generates discovery response for full catalog
• Round-trip preserves name, description, tags, namespace
• Tests cover conversion, round-trip, and edge cases
• Documentation updated in docs/integration_a2a.md

File Paths

• src/contextweaver/adapters/a2a.py (edit)
• tests/test_adapters.py (edit)
• docs/integration_a2a.md (edit)

Cross-references

• Related to docs: add LangChain + LangGraph integration guide + examples #80 (A2A skill-card adapter)