feat: machine-readable verification output (JSON) for CI

[omsherikar]

Problem

format_verification_result in refactron/verification/report.py is Rich console output only. CI systems, bots, and merge gates need structured data (exit code alone is not enough for dashboards or annotations).

Impact

Harder to integrate refactron verify into GitHub Actions, generic scripts, or parent tools without scraping terminal text.

Suggested direction

• Add --json (or --format json) to refactron verify that prints a stable JSON object: overall status, blocking_reason, per-check CheckResult fields (check_name, passed, blocking_reason, confidence, duration_ms, details).
• Keep schema versioned (e.g. "schema_version": 1) so consumers can evolve safely.
• Optional: helper VerificationResult.to_json_dict() to avoid duplicating field lists.

References

• refactron/cli/verify.py
• refactron/verification/result.py