Scratch directory cleaned up after run — no way to inspect artifacts

[schgoo]

Problem

After a successful harness run, the scratch directory (generated Cargo project, compiled binary, trace output) is not preserved. When debugging failures during development, you need to:

1. Run the harness
2. Hope the scratch dir still exists (it does on failure, but not success)
3. Or add breakpoints / print statements to the harness code itself

Example

After run_spec("specs/edm.primitive_types.spec.yaml") completes successfully:

• Generated main.rs — gone
• traces.json — gone
• Compiled sg-runner binary — gone (unless cached from a previous failed run)

To inspect actual traces, I had to find the cached binary from a prior run and manually invoke it:

/path/to/sg-runner /tmp/traces.json

Suggestion

Add a mechanism to preserve artifacts:

• SPECGATE_KEEP_ARTIFACTS=1 env var
• Or a run_spec_opts variant with a keep_artifacts: bool parameter
• At minimum, keep traces.json on success (it's small and valuable for debugging)