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>

internal/linter/linter.go

@@ -62,6 +62,7 @@ func Lint(p *model.Pipeline, skipped map[string]bool) []Finding {
 	findings = append(findings, checkWorkflow(p)...)
 	findings = append(findings, checkJobs(p)...)
 	findings = append(findings, checkNeeds(p, skipped)...)
+	findings = append(findings, checkRulesNeeds(p, skipped)...)
 	findings = append(findings, checkDependencies(p, skipped)...)
 	findings = append(findings, checkVariableRefs(p)...)
 	findings = append(findings, checkRulesIfReachability(p)...)