fix: resolve nullability warnings, add CI/CD and security workflows, harden .gitignore

- Add nullable annotations across discovery interfaces, dynamic query
  models, and filter/aggregate types to eliminate CS8600-series warnings
- Replace unsafe cast in DynamicQueryHandlerBase with pattern match
- Add CI workflow (build --warnaserror + test on JP branch)
- Add weekly security vulnerability scan workflow
- Extend .gitignore with secret/credential patterns (.env, *.key, secrets/, credentials.json)

Co-Authored-By: Svrnty Inc. <eng@svrnty.com>

Svrnty.CQRS.DynamicQuery/Discover/DynamicQueryMeta.cs

@@ -22,7 +22,7 @@ public class DynamicQueryMeta(Type queryType, Type serviceType, Type queryResult
         }
     }
 
-    public Type ParamsType { get; internal set; }
-    public string OverridableName { get; internal set; }
+    public Type? ParamsType { get; internal set; }
+    public string? OverridableName { get; internal set; }
 }