Resume now follows the same subprocess pattern as run: look up run
directory by ID prefix, validate checkpoint exists, clean stale
artifacts, reset status to Submitted, spawn _run_engine --resume, and
attach. This eliminates ~1600 lines of duplicated env/sandbox setup
from resume.rs.
Key changes:
- operations::start() and operations::resume() take run_dir instead
of Persisted, loading state from disk internally
- run_engine() builds RunOptions from RunRecord on disk, so callers
no longer extract record fields manually
- StartOptions flattened (no more nested InitOptions)
- FabroError::Precondition variant for start/resume guard checks
- _run_engine accepts --resume flag to dispatch to resume path
- operations::restore removed (no longer needed)
- Resume CLI stripped to just <RUN_ID> + --detach
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>