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>

.github/workflows/ci.yml

@@ -0,0 +1,27 @@
+name: CI
+
+on:
+  push:
+    branches: [JP]
+  pull_request:
+    branches: [JP]
+
+jobs:
+  build-and-test:
+    name: Build & Test
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: '10.0.x'
+
+      - name: Restore dependencies
+        run: dotnet restore
+
+      - name: Build (warnings as errors)
+        run: dotnet build --no-restore --warnaserror
+
+      - name: Test
+        run: dotnet test --no-build --verbosity normal