feat: offline-verifiable decision receipts (Ed25519 + JCS) #1499

Overview

Implement offline-verifiable decision receipts as a first-party AGT capability. Every tool call an agent makes should produce a cryptographic receipt that third parties can verify without access to the original infrastructure.

Background

This feature was previously contributed by an external party but was removed in PR #1498 after the contributor was flagged for credential laundering by AGT's governance tools. The underlying concept has genuine value and should be reimplemented properly.

Requirements

• Receipt format: Per-tool-call Ed25519 signatures over JCS-canonical (RFC 8785) payloads
• Hash chaining: Receipts linked via parent hashes for insertion detection
• Offline verification: CLI tool to verify receipt chains without network access
• Cedar integration: Bind Cedar policy decisions to receipt payloads
• Audit log integration: Two-layer integrity with AGT's existing AuditLog
• SLSA provenance: Optional emission as SLSA provenance metadata

Standards Alignment

• Ed25519 (RFC 8032)
• JSON Canonicalization Scheme (RFC 8785)
• IETF draft-farley-acta-signed-receipts
• SLSA provenance predicates

Deliverables

• Core receipt signing/verification module in agent-governance-python
• CLI verification tool (pure Python, no external dependencies)
• Tutorial covering receipt creation, verification, chaining, and CI integration
• Quickstart example
• Unit tests

Labels

enhancement, good first issue