docs: add complete OpenAPI 3.0 spec for miner HTTP endpoints (closes #23)

[gramseostudio-dev]

What does this PR do?

This PR introduces comprehensive OpenAPI 3.0.3 documentation for the Engram Miner API, specifying endpoints and the cryptographic security schemes needed for validator-miner communication.

Closes #23

Changes

• docs/openapi.yaml: Added complete OpenAPI 3.0.3 specification mapping key endpoints (/ChallengeSynapse, /stats) and required authentication models (Sr25519Signature, HmacChallengeProof).
• docs/changelogs/changelog-engram-openapi.md: Documented the change tracking and feature context for the openapi branch lifecycle.

How to test

Since this is purely documentation and static specs, reviewers can verify the schema syntax and endpoints visually using any standard OpenAPI/Swagger viewer or directly via the raw file in GitHub.

Checklist

• Tests added or updated for the changed behavior
• All existing tests pass (python -m pytest tests/ -q)
• Security-sensitive changes described without exposing exploit details
• No unrelated formatting, refactors, or scope creep

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[gramseostudio-dev]

Hi @Dipraise1! I've successfully mapped out the complete OpenAPI 3.0.3 specification covering the miner HTTP endpoints (/ChallengeSynapse and /stats), along with the necessary cryptographic security schemes (Sr25519Signature and HmacChallengeProof).

The spec is clean and ready for review. Let me know if you need any adjustments or further refinements!