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>

FEATURES.md

@@ -69,6 +69,7 @@ description, bad-YAML example, and fix.
 | GL029 | ERR | Circular dependency in `needs:` graph |
 | GL030 | ERR | `dependencies:` references a job that doesn't exist |
 | GL031 | ERR | `dependencies:` references a job in the same or a later stage |
+| GL044 | ERR/WARN | `rules:needs:` references a job that doesn't exist (WARN when `optional: true`) |
 
 ### Expression & reachability