Adds Jenkins as the third platform alongside GitLab CI and GitHub Actions.
Six new rules (JKN-PIPE-001, JKN-IAM-001, JKN-RUN-001, JKN-RUN-002,
JKN-SC-001, JKN-SC-002), four built-in policies (POL-JKN-001..004), and a
hand-rolled Groovy-aware parser that handles brace + string-aware block
extraction at depth 0.

Total: 37 rules across 3 platforms, 17 built-in policies, 249 tests passing.

See CHANGELOG.md for full notes.