Self-update

Criterion

A tool reports whether a newer version is available without changing anything, and that check has a JSON form.

Rationale

Agents and CI need a read-only answer to “is this current?” separate from installing a release. A JSON form lets automation gate on the answer.

Testable assertions

• update.check-readonly (required): update --check reports status and exits 0 without modifying the binary.
• update.check-json (recommended): update --check --json emits valid JSON.