[agent-harness] P12.1 CONTRIBUTING - change log line for contract + capabilities

[ManintheCrowds]

Phase 1 — P12.1

Outcome: CONTRIBUTING.md requires a release-note or changelog line when ARCHITECTURE_REST_CONTRACT or /api/capabilities changes (same spirit as existing API checklist).

Seed: P12 backlog

Acceptance criteria

• CONTRIBUTING explicitly mentions changelog / release notes for contract + capabilities drift.
• Aligns with existing numbered API steps (no duplicate bureaucracy).

Blocks: P12.2 (optional CI follow-up)

[ManintheCrowds]

Repo naming: Canonical policy and checklist — docs/engineering/OPENGRIMOIRE_NAMING_AND_URLS.md (GitHub ManintheCrowds/OpenGrimoire, local folder OpenGrimoire).