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>

internal/linter/linter.go

@@ -53,6 +53,7 @@ func Lint(p *model.Pipeline) []Finding {
 	findings = append(findings, checkJobs(p)...)
 	findings = append(findings, checkNeeds(p)...)
 	findings = append(findings, checkDependencies(p)...)
+	findings = append(findings, checkVariableRefs(p)...)
 	return findings
 }