feat(api): OpenAPI 3.0 specification

[xe-nvdk]

Summary

Write a formal OpenAPI 3.0 specification for all Memtrace REST API endpoints. This unlocks auto-generated SDK clients, Postman collections, and ChatGPT GPT Actions.

Scope

Cover all endpoints documented in docs/api.md:

• Health: GET /health, GET /ready
• Memories: POST /api/v1/memories, GET /api/v1/memories, GET /api/v1/memories/:id
• Agents: Full CRUD + stats
• Sessions: Full lifecycle + POST /api/v1/sessions/:id/context
• Search: POST /api/v1/search

Include authentication schema (x-api-key header with mtk_ prefix).

Downstream

• Enables ChatGPT GPT Action (#issue)
• Enables auto-generated clients for any language
• Enables Postman/Insomnia collections