feat(01-01): create three-project .NET 9 solution scaffold

- ChatAgent.sln at repo root with Client, Api, Shared projects
- Blazor WASM client, ASP.NET Core Web API, shared class library
- Both Client and Api reference Shared project
- Removed template boilerplate (WeatherForecast, Class1)
- Added .NET ignores to .gitignore
- Pinned .NET 9 SDK via global.json

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -216,3 +216,14 @@ pyvenv.cfg
 .venv
 pip-selfcheck.json
 
+# ---> .NET / C#
+bin/
+obj/
+*.user
+*.suo
+.vs/
+.idea/
+*.swp
+**/wwwroot/_framework/
+*.DotSettings.user
+