Exit codes and JSON

code	meaning
0	success
1	error. The message says what went wrong, and a hint: line says what to do.
2	usage error: unknown command, missing or bad argument
3	a check did not pass: doctor found a problem, proof status is not proven, proof test failed, selftest failed

keelyard run exits with the exit code of the command it ran.

With --json, a command prints exactly one JSON object to stdout and nothing else. The object always has ok and exit:

{"proven": false, "missing": ["Requirement #2 not verified: unknown coupons raise ValueError"], "ok": false, "exit": 3}

Errors keep the same shape:

{"ok": false, "exit": 1, "error": "no step 42 (steps 1-4 exist).", "hint": "run `keelyard rewind list`."}