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>

README.md

@@ -298,6 +298,12 @@ Every finding includes a stable rule ID (e.g. `GL003`) that can be used to filte
 | GL030 | ERROR | `dependencies:` references a job that does not exist |
 | GL031 | ERROR | `dependencies:` references a job in the same or a later stage |
 
+### Expression validation
+
+| ID | Severity | Rule |
+|----|----------|------|
+| GL032 | WARNING | `rules:if:` references `$VAR` not declared in `variables:` (pipeline, job, or `workflow:rules:variables:`) — may be a false positive for variables set in GitLab CI/CD project settings |
+
 ### Hidden jobs (templates)
 
 Jobs whose name starts with `.` are treated as reusable templates and skipped for most rules. This matches GitLab's own behaviour.