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/rules.go

@@ -107,4 +107,11 @@ const (
 
 	// GL031: dependencies: references a job in the same or a later stage.
 	RuleDependencyStage = "GL031"
+
+	// ── Expression validation ────────────────────────────────────────────────
+
+	// GL032: rules:if: references a variable not declared in pipeline variables:,
+	// the job's own variables:, or any workflow:rules:variables: block.
+	// May be a false positive for variables set in GitLab CI/CD project settings.
+	RuleUndeclaredVariable = "GL032"
 )