feat(linter): rules:needs: validation (GL044)

Add GL044 to validate jobs listed in rules:needs: overrides (GitLab CI
16.4+). rules:needs: lets a specific rule override the job's top-level
needs: list; any referenced job must exist in the pipeline. Unknown jobs
produce an error; optional: true entries produce a warning, matching the
GL027 behaviour for top-level needs:. Cross-pipeline needs and skipped
jobs (when a context is active) are excluded from checking.

Implementation:
- model.Rule gains a Needs []any field (yaml:"needs")
- checkRulesNeeds(p, skipped) added to needs.go; wired into Lint
- GL044 / RuleRulesNeedsUnknown added to rules.go and explain.go
- 6 unit tests + 2 testdata fixtures; task validate updated

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -6,7 +6,7 @@
 
 <p align="center">
   <a href="LICENSE"><img src="https://img.shields.io/badge/license-Apache%202.0-blue.svg" alt="License"></a>
-  <a href="CHANGELOG.md"><img src="https://img.shields.io/badge/release-v0.2.23-blue.svg" alt="Release"></a>
+  <a href="CHANGELOG.md"><img src="https://img.shields.io/badge/release-v0.2.24-blue.svg" alt="Release"></a>
 </p>
 
 > **Disclaimer:** This tool was built through iterative AI-assisted development with [Claude](https://claude.ai). It is experimental, incomplete, and not intended for production use. Coverage of GitLab CI keywords is best-effort and may lag behind GitLab's evolving spec. Use it at your own discretion — no correctness guarantees are made. Contributions and bug reports are welcome.