docs: add OpenAPI 3 spec for miner HTTP API + Redoc reference + CI validation

[laeeH]

Summary

This PR adds a complete OpenAPI 3.0 specification for the Engram Miner HTTP API (served on port 8091) and a rendered Redoc reference page.

Changes

1. docs/openapi.yaml — Full OpenAPI 3.0 spec covering all miner endpoints:

   • Public: /health, /stats, /metagraph
   • Authenticated (sr25519 signed): /IngestSynapse, /QuerySynapse, /ChallengeSynapse, /retrieve/{cid}
   • Namespace & key mgmt: /namespace, /AttestNamespace, /attestation/{namespace}, /KeyShareSynapse, /KeyShareRetrieve
   • Repair & replication: /RepairSynapse, /list
   • Merkle proofs: /commitment, /prove-memory

2. docs/api-reference.html — Interactive Redoc page for browsing the API

3. docs/miner.md — Added API Reference section with links

4. .github/workflows/ci.yml — Added openapi-lint job using Spectral to validate the spec on every PR

Related Issue

Addresses #23 (OpenAPI spec for miner HTTP endpoints)

Bounty

This PR addresses the bounty listed in ROADMAP.md for generating an OpenAPI spec from the miner's route definitions.

[vercel]

Someone is attempting to deploy a commit to the praise's projects Team on Vercel.

A member of the Team first needs to authorize it.