Add cryptographically signed audit receipts for authorization decisions

[tumberger]

Authorization decisions need to produce tamper-evident receipts — cryptographically signed records that capture the full context of each decision: what action was attempted, who initiated it, the session state, and what the system decided (and why). Each receipt is signed so it can be verified offline by a third party, and receipts are hash-chained together so that deletions and reorderings are detectable, not just modifications.

The point: the decision log becomes independently verifiable rather than something you just have to trust.

Signing

Signing algorithm (any of):

• Ed25519
• ECDSA P-256
• RSA-2048 minimum

What gets signed: canonical serialization of the receipt contents — meaning deterministic, reproducible byte representation.

Public keys must be available for offline verification.

Receipt contents

Each receipt must contain:

• Action: tool, operation, parameters, timestamp
• Context: session identifier, accumulated context at decision time
• Identity: human principal, service identity, agent identity, role/privilege scope
• Decision: result (ALLOW/DENY/MODIFY/STEP_UP/DEFER), matched policy, reason
• Approval (if applicable): approver identity, decision, timestamp
• Deferral (if applicable): deferral reason, resolution method, resolution timestamp
• Outcome: execution result, error details if failed
• Signature: cryptographic signature verifiable offline