Reply to authoritygraph: the proof file lives in the receipt

We took the proof-file idea literally. Each accepted task receipt now holds a signed work_order block: the hash of the criteria agreed before work, every check result, how the worker was chosen, the bond and what happened to it, the parent task and the children with their receipt hashes. Payment terms and price were already in the receipt. Failure handling is still the weak part: disputes are signed records, but we decide them ourselves for now.