feat(flutter-cursor-templates): add /debug, /verify, /explain skills and goldens.

Wire debug-issue, verify-change, and explain-code into the universal resolver with reasons, extend AGENTS.md slash list, cross-link build Phase 3 and Phase 6, refresh generator goldens for three stacks, and note cross-skill links in the build skill design spec.

Co-authored-by: Cursor <cursoragent@cursor.com>

flutter-cursor-templates/generator/src/resolver.dart

@@ -73,6 +73,9 @@ class Resolver {
       'skills/scaffold-screen',
       'skills/generate-tests',
       'skills/build',
+      'skills/debug-issue',
+      'skills/verify-change',
+      'skills/explain-code',
     ]);
     if (brief.apiDocsFormat != 'none') files.add('skills/generate-api-client');
     if (brief.flavors.length > 1)      files.add('skills/create-flavor');
@@ -127,6 +130,9 @@ class Resolver {
     if (key.contains('migration'))      return 'state_management is GetX — migration guidance included';
     if (key.contains('security-agent')) return 'scale: ${brief.scale} or auth is configured';
     if (key == 'skills/build')           return 'Always included — universal TDD-first feature implementation command';
+    if (key == 'skills/debug-issue')     return 'Always included — structured bug triage and evidence-first debugging';
+    if (key == 'skills/verify-change')  return 'Always included — pre-PR verification checklist without full /build lifecycle';
+    if (key == 'skills/explain-code')    return 'Always included — explain-only walkthrough of code paths and stack behavior';
     if (key.contains('api-client'))     return 'api_docs.format is set';
     if (key.contains('realtime'))       return 'features.special contains realtime';
     if (key.startsWith('root/'))        return 'Repo-level companion files';