feat: post-execution audit trail pattern to complement AgentGovernancePlugin

Now that #141 (AgentGovernancePlugin) is merged, the pre-execution governance loop is closed: policy load → tool-call decision → allow/review/block.

The natural next question is post-execution: once the tool call is allowed and executed, is there a standard pattern for recording what actually happened in a tamper-evident way?

The gap: AgentGovernancePlugin captures the decision. Nothing currently captures the outcome — tool name, args hash, output hash, timestamp — in a form that an external auditor can verify independently.

**Proposed pattern:** an optional `audit_backend` parameter on `AgentGovernancePlugin` (or a companion hook) that receives a structured post-execution record after each allowed tool call. Implementations could range from local JSONL to on-chain anchoring (e.g. Mycelium Trails).

This would complete the loop: pre-execution policy check → execution → post-execution evidence. Both sides independently verifiable.

Happy to contribute a spec or draft implementation if there's interest.