receipt/v0 package

Package receipt is an append-only log of agent execution receipts.

The thesis: provenance is not correctness. A chain can prove *who* committed to an output, *when*, that the bytes have not changed since, and *what* was staked or attested — but it cannot, by itself, prove the output was any good. Correctness comes from an external validation mechanism.

So a receipt stores commitments (hashes) to the task, inputs, outputs, tool calls and runtime, never the raw data — that lives off-chain. The realm then lets independent validators attach attestations: "I re-ran this and it reproduced", "the tests pass", "the output matches the commitment", or "rejected". The receipt's trust level is exactly the sum of the attestations it has collected, and nothing more.