RFC: Cryptographic decision receipts for Conductor task execution

## Problem

Conductor orchestrates complex workflows with durable execution guarantees, but the audit trail of task-level decisions is stored in the system's database without cryptographic integrity guarantees. For enterprise deployments in regulated industries (financial services, healthcare, government), compliance teams need to independently verify that no workflow decision has been altered after execution.

## Proposal

Integrate Ed25519 receipt signing into Conductor's task execution lifecycle. Each task completion would emit a signed receipt capturing the decision, input/output hashes, and worker identity:

```json
{
"task": "process_transaction",
"decision": "completed",
"worker_id": "payment-processor-3",
"input_digest": "sha256:a3f8...",
"output_digest": "sha256:b7c2...",
"signature": "Ed25519:...",
"workflow_id": "wf-12345",
"timestamp": "2026-04-05T14:00:00Z"
}
```

This would enable:
- **Tamper-evident audit trails** for workflow execution history
- **Offline verification** of workflow decisions without access to Conductor's database
- **Compliance export** as self-contained, cryptographically verifiable bundles

## Reference implementation

[protect-mcp](https://github.com/scopeblind/scopeblind-gateway) already implements this pattern for MCP tool calls. The receipt format is standardized in an [IETF Internet-Draft](https://datatracker.ietf.org/doc/draft-farley-acta-signed-receipts/) and has active integrations with Mission Control, Cedar for Agents, and Microsoft's Agent Governance Toolkit.

Happy to discuss integration architecture and contribute a PR.

Start by reviewing Conductor's task execution lifecycle and the protect-mcp reference implementation linked in the issue. Define the receipt format, signing and verification boundaries, key handling, and compliance export behavior before identifying the affected components; done means task decisions produce independently verifiable, tamper-evident receipts.

Written by the indexing model from the issue text.