[docs] OpenAPI 3 spec for miner HTTP endpoints (#23)

[5AIBountyHunter]

Summary

Adds an OpenAPI 3.0.3 specification covering all 24 registered miner HTTP endpoints, including auth scheme documentation and a CI validation workflow.

Changes

New files:

• docs/openapi.yaml — Complete OpenAPI 3 spec covering:
  • Ingest, Query, Challenge, Namespace, Attestation operations
  • KeyShare store/retrieve, Repair, List, Health, Stats
  • Chat history CRUD, Conversations CRUD
  • Commitment, Prove-memory, Wallet-stats, Metagraph, Metrics
  • sr25519 signed-challenge auth scheme documentation
• .github/workflows/openapi-check.yml — CI workflow to validate spec stays in sync

Details

• All 24 endpoints synced from neurons/miner.py route registrations
• Request/response schemas documented for key operations
• Auth section explains both signature-based and key-based modes
• Spec validated against OpenAPI 3.0.3 schema

Closes #23

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[5AIBountyHunter]

Hi @Dipraise1, this PR adds the OpenAPI spec requested in #23. I also have PR #37 open for the TypeScript SDK. Both are related — the OpenAPI spec is useful for auto-generating the TS client.

Please review when you have a chance. Happy to adjust!