Initial WebhookServer implementation

Add the .NET 8 solution scaffolded against PLAN.md. Three projects share
WebhookServer.Core (models, auth, execution, storage, IPC, callbacks)
and WebhookServer.Service hosts an embedded Kestrel listener plus the
named-pipe admin server. WebhookServer.Gui is a thin MVVM client over
the pipe. Includes 25 unit tests covering HMAC verification, bearer
auth, IP allowlist parsing, arg-template rendering, DPAPI round-trip,
and the encrypt-on-save config store.

Install/uninstall PowerShell scripts default to LocalSystem and accept
a domain user or gMSA via -ServiceAccount.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/WebhookServer.Service/WebhookServer.Service.csproj

@@ -0,0 +1,21 @@
+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <UserSecretsId>dotnet-WebhookServer.Service-57f4579b-6131-4fab-a6ad-2865b038cc2e</UserSecretsId>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.Hosting" Version="8.0.1" />
+    <PackageReference Include="Microsoft.Extensions.Hosting.WindowsServices" Version="8.0.1" />
+    <PackageReference Include="Serilog.AspNetCore" Version="8.0.3" />
+    <PackageReference Include="Serilog.Sinks.Async" Version="2.1.0" />
+    <PackageReference Include="Serilog.Sinks.File" Version="6.0.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\WebhookServer.Core\WebhookServer.Core.csproj" />
+  </ItemGroup>
+</Project>