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>

testdata/rules_needs_invalid.yml

@@ -0,0 +1,15 @@
+stages:
+  - build
+  - test
+
+build-job:
+  stage: build
+  script: make
+
+test-job:
+  stage: test
+  script: make test
+  rules:
+    - if: '$CI_COMMIT_BRANCH == "main"'
+      needs: [build-job, nonexistent-job]   # nonexistent-job does not exist → GL044
+    - when: on_success

testdata/rules_needs_valid.yml

@@ -0,0 +1,19 @@
+stages:
+  - build
+  - test
+
+build-job:
+  stage: build
+  script: make
+
+lint-job:
+  stage: build
+  script: make lint
+
+test-job:
+  stage: test
+  script: make test
+  rules:
+    - if: '$CI_COMMIT_BRANCH == "main"'
+      needs: [build-job, lint-job]   # both exist → clean
+    - when: on_success