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>

src/ChatAgent.Api/Program.cs

@@ -0,0 +1,23 @@
+var builder = WebApplication.CreateBuilder(args);
+
+// Add services to the container.
+
+builder.Services.AddControllers();
+// Learn more about configuring OpenAPI at https://aka.ms/aspnet/openapi
+builder.Services.AddOpenApi();
+
+var app = builder.Build();
+
+// Configure the HTTP request pipeline.
+if (app.Environment.IsDevelopment())
+{
+    app.MapOpenApi();
+}
+
+app.UseHttpsRedirection();
+
+app.UseAuthorization();
+
+app.MapControllers();
+
+app.Run();