Provenance manifests

Every run writes outputs/<exp>/<run-id>/manifest.json with:

• labflow_version, schema_version
• experiment, run_id, timestamp_utc
• git_commit, git_dirty
• config_rendered, config_hash, seed
• env (backend, lockfile_hash, Python version, packages)
• hardware (arch, CPU, memory, GPU)
• resources_requested, resources_used
• slurm_job_id, container_hash
• outputs (list of output files), returned (function return value if dict)

Replay a run → re-hash config + env + git + seed → compare to this manifest.