feat(allowlist): add Crystal language support

Sub-issue of #470 — claiming the Crystal language group.

Will add:
- `.cr` to the supported file types allowlist
- `**/spec/**/*_spec.cr` to default exclude patterns (Crystal's conventional spec dir)
- test cases in `allowed_ext_test.go`
- a dedicated `crystal.md` review rule doc, registered in `system_rules.json`
- a resolution test case

Will follow up with a focused PR.

Start with allowed_ext_test.go and inspect the existing language-support patterns. Add .cr handling, the Crystal spec-directory exclusion, the crystal.md review rule registered in system_rules.json, and the resolution test case; run the affected tests to verify all requested Crystal support is covered.

Written by the indexing model from the issue text.