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.Abstractions/IDynamicQuery.cs

@@ -20,10 +20,10 @@ public interface IDynamicQuery<TSource, TDestination, out TParams> : IDynamicQue
 
 public interface IDynamicQuery
 {
-    List<IFilter> GetFilters();
-    List<IGroup> GetGroups();
-    List<ISort> GetSorts();
-    List<IAggregate> GetAggregates();
+    List<IFilter>? GetFilters();
+    List<IGroup>? GetGroups();
+    List<ISort>? GetSorts();
+    List<IAggregate>? GetAggregates();
     int? GetPage();
     int? GetPageSize();
 }

Svrnty.CQRS.DynamicQuery.Abstractions/IDynamicQueryParams.cs

@@ -3,5 +3,5 @@
 public interface IDynamicQueryParams<out TParams>
     where TParams : class
 {
-    TParams GetParams();
+    TParams? GetParams();
 }