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/rules.go

@@ -154,4 +154,8 @@ const (
 	// pipeline has no 'default:' block, making the declaration a no-op. Also fires
 	// when 'inherit: default: [list]' names fields not set in the default: block.
 	RuleInheritNoDefault = "GL043"
+
+	// GL044: a rules:needs: entry references a job that does not exist in the pipeline.
+	// rules:needs: overrides the top-level needs: when a specific rule matches (GitLab CI 16.4+).
+	RuleRulesNeedsUnknown = "GL044"
 )