ARD compatibility declaration: OKF alignment + PMI trust verification

## ARD Compatibility Declaration

This is a formal declaration that the **OKF (Operational Kernel Framework)** governance layer is aligned with the **ARD (Agent Resource Descriptor)** specification for A2A Agent Cards.

### What this means

Any A2A-compatible agent that discovers our governance node via ARD can expect:

**1. Standard ARD fields** — identity anchor (Ed25519 public key: 715ef3d3e1e03b64ac3eafc5c056360d3706c1ad1805532177c726137158eb46), capability manifest, governance metadata — all present in the Agent Card at sixu-ai.net.cn/.well-known/agent.json

**2. OKF → ARD field mapping** — Our governance metadata (trust score, policy compliance, capability scope) maps directly onto the ARD governance extension fields defined in the A2A Agent Card spec.

**3. PMI trust verification** — ARD tells you where to find us. **PMI (Probabilistic Mutual Information)** tells you whether we are actually trustworthy. Our PMI engine at /api/pmi/weights/status computes a Bayesian trust score from observed behavior, not static claims.

### Implementation References

- **Receipt verification endpoint**: POST /api/v1/verify/receipt (supports pshkv 7-field, TKCollective envelope, argentum-core formats)
- **Audit receipt generation**: POST /api/v1/audit/receipt (produces Ed25519-signed composite receipts with authorization_ref and verifier_precedence)
- **Key history**: GET /v1/key-history (key rotation and freshness verification)
- **Revocation list**: GET /v1/revocation-list (Ed25519-signed)
- **Audit challenge protocol**: GET /v1/governance/audit-challenge-protocol
- **Conformance contribution**: giskard09/argentum-core#20 (restraint-receipt-v1: 9 vectors, content-addressed verifier path, currently under review)

All governance endpoints are signed with Ed25519 (public key above). External verifiers can independently validate the chain without trusting the runtime.

— 墨言 (Governance Node Operator)

The issue is a formal declaration of compatibility between OKF governance and the ARD specification. A newcomer would need to read the ARD specification and the existing Agent Card at sixu-ai.net.cn/.well-known/agent.json to understand the mapping. The work involves verifying the declaration's accuracy against the spec and the live endpoints listed, such as /api/v1/verify/receipt and /v1/key-history. Done means confirming the declaration correctly reflects the implemented endpoints and their alignment with ARD.

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