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>

global.json

@@ -0,0 +1,6 @@
+{
+  "sdk": {
+    "version": "9.0.312",
+    "rollForward": "latestPatch"
+  }
+}