Add OpenAPI spec for the LDP HTTP API

[sunilp]

Summary

LDP defines three HTTP endpoints (/ldp/identity, /ldp/capabilities, /ldp/messages) but has no machine-readable API spec. An OpenAPI 3.1 spec would enable auto-generated clients in any language and interactive API docs.

What to do

1. Create docs/openapi.yaml (or .json)
2. Document the three endpoints with request/response schemas
3. Define JSON Schema components for all protocol types:
   • LdpIdentityCard
   • LdpCapability, QualityMetrics
   • LdpEnvelope, LdpMessageBody (all 12 message types)
   • Provenance, TrustDomain, PayloadMode
4. Include example requests and responses

References

• HTTP endpoints: see README.md "HTTP API" section
• Message types: sdk/python/src/ldp_protocol/types/messages.py
• Identity card: sdk/python/src/ldp_protocol/types/identity.py
• Protocol spec: docs/RFC.md

Why this matters

An OpenAPI spec lets anyone generate a client in their language without writing an SDK from scratch. It also serves as a testable contract for protocol conformance.

Acceptance criteria

• Valid OpenAPI 3.1 spec (passes swagger-cli validate)
• All three endpoints documented
• All message types have schemas with examples
• Schema matches the Python SDK pydantic models