ReproGate v0.1.3

What changed

• Added reusable GitHub Action outputs: labels, missing, missing_count, ready, summary, and result_json.
• Added a ready-check --github-output option for composite action integration.
• Added self-tests for Action outputs.

Verify

• go test ./...
• ready-check --github-output local smoke test
• GitHub Actions: reprogate and action-self-test passed on main