feat(linter): GL033 static dead-rules detection

Add rule GL033 that warns when every rule in a job's rules: block has
an explicit when: never, making the job permanently excluded from any
pipeline run. This is a pure static check — no if: evaluation or context
required. Only rules with literal when: never trigger it; rules with no
when: (defaults to on_success), when: manual, when: always, or
when: on_failure are treated as reachable.

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

internal/linter/rules.go

@@ -114,4 +114,8 @@ const (
 	// 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"
+
+	// GL033: every rule in a job's rules: block has when: never, so the job
+	// can never be included in any pipeline run.
+	RuleDeadRules = "GL033"
 )