feat: optional independent signed catalog authority receipt

[altrudev]

Summary

Adds an opt-in, independently signed Ed25519 correspondence receipt binding the Agent Manifest signing digest, agent identity, policy bundle hash, complete sealed cMCP catalog hash, and Agent Manifest Merkle root. Preserves upstream #713 native tool projection and verification; no replacement or bypass of SDK validation.

Assurance

• Domain-separated signed receipt, strict canonical fields and validity interval
• Fail-closed startup when configured receipt, measurements or signer are invalid
• Native upstream catalog verification remains mandatory
• Operator deployment/rotation and limitation documentation
• Local VPS: pytest -q tests/unit tests/integration: 2382 passed, 6 skipped, 4 deprecation warnings; git diff --check clean
• Six skips: real Azure SEV-SNP fixture (2), real TDX quote (2), optional Agent Governance Toolkit package (2)

Review requests

Please review whether an independent issuer receipt is useful alongside native Merkle binding; specifically JSON/COSE signing preimages, trust anchor management, optional deployment semantics, and cryptographic canonicalization. This is a draft for independent maintainer review, not a claim of hardware attestation or release approval.

No private keys or proprietary runtime implementation are included.

[github-actions]

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