[tool] Update skills_lint to published version ^0.5.1

- Update tool/generator/pubspec.yaml to depend on published skills_lint: ^0.5.1
- Remove custom LastModifiedRule and its enforcement from lint_skills_test.dart
- Verify all 100 tests and static analysis pass cleanly

Author: reidbaker-agent

tool/generator/pubspec.yaml

@@ -26,10 +26,6 @@ dev_dependencies:
   build_verify: ^3.1.0
   coverage: ^1.15.0
   lints: ^6.0.0
-  skills_lint:
-    git:
-      url: https://github.com/google/skills_lint.dart.git
-      path: packages/skills_lint
-      ref: e6e695e1550f81342fe5acd4dbe65040b5aa44c3
+  skills_lint: ^0.5.1
   test: ^1.25.6
 

tool/generator/test/custom_skill_rules/last_modified_rule.dart

@@ -9,8 +9,6 @@ import 'package:path/path.dart' as p;
 import 'package:skills_lint/skills_lint.dart';
 import 'package:test/test.dart';
 
-import 'custom_skill_rules/last_modified_rule.dart';
-
 void main() {
   test('Run skills linter', () async {
     final lintErrors = <String>[];
@@ -59,10 +57,7 @@ void main() {
         reason: 'Configuration directoryConfigs should not be empty.',
       );
 
-      final isValid = await validateSkills(
-        config: config,
-        customRules: [LastModifiedRule()],
-      );
+      final isValid = await validateSkills(config: config);
       expect(
         isValid,
         isTrue,