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/explain.go

@@ -816,4 +816,41 @@ my-job:
 my-job:
   script: echo hi`,
 	},
+
+	RuleRulesNeedsUnknown: {
+		Title:    "'rules:needs:' references unknown job",
+		Severity: Error,
+		Description: "A job listed in a rule's 'needs:' override does not exist in " +
+			"the pipeline. 'rules:needs:' (GitLab CI 16.4+) overrides the top-level " +
+			"'needs:' list when that specific rule matches. GitLab will refuse to " +
+			"create the pipeline if any referenced job is missing.",
+		Example: `stages: [build, test]
+
+build:
+  stage: build
+  script: make
+
+test:
+  stage: test
+  script: make test
+  rules:
+    - if: $CI_COMMIT_BRANCH == "main"
+      needs: [build, lint]   # lint does not exist`,
+		Fix: `stages: [build, test]
+
+lint:
+  stage: build
+  script: make lint
+
+build:
+  stage: build
+  script: make
+
+test:
+  stage: test
+  script: make test
+  rules:
+    - if: $CI_COMMIT_BRANCH == "main"
+      needs: [build, lint]`,
+	},
 }