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/model/pipeline.go

@@ -89,6 +89,7 @@ type Rule struct {
 	Changes   any            `yaml:"changes"`   // []string or {paths,compare_to} map
 	Exists    any            `yaml:"exists"`    // []string or map form
 	Variables map[string]any `yaml:"variables"` // set/override variables when rule matches (GitLab CI 15.0+)
+	Needs     []any          `yaml:"needs"`     // override needs: when this rule matches (GitLab CI 16.4+)
 }
 
 // ReservedKeys are top-level GitLab CI keys that are NOT job definitions.