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.Core/Execution/IExecutor.cs

@@ -0,0 +1,8 @@
+using WebhookServer.Core.Models;
+
+namespace WebhookServer.Core.Execution;
+
+public interface IExecutor
+{
+    Task<ExecutionResult> RunAsync(EndpointConfig endpoint, ExecutionContext ctx, CancellationToken ct);
+}