feat(linter): add GL032 variable reference validation in rules:if:

Warn when a $VAR or ${VAR} reference in a rules:if: expression is not
declared in pipeline variables:, the job's own variables:, or any
workflow:rules:variables: block. Predefined GitLab CI namespaces (CI_*,
GITLAB_*, FF_*, RUNNER_*, TRIGGER_*, CHAT_*) are always exempt.

Each undeclared variable is reported at most once per job. The finding
is a WARNING (not an error) because variables may also be set in GitLab
CI/CD project settings, which are invisible to glint at lint time.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Taskfile.yml

@@ -73,6 +73,8 @@ tasks:
         ignore_error: false
       - cmd: ./{{.BINARY}} check --branch feat/x testdata/workflow_vars.yml
         ignore_error: false
+      - cmd: ./{{.BINARY}} check testdata/variable_refs.yml
+        ignore_error: false
       - cmd: ./{{.BINARY}} check samba-testdata/.gitlab-ci.yml
         ignore_error: false
       - cmd: ./{{.BINARY}} check samba-testdata/.gitlab-ci-coverage.yml