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/Callbacks/CallbackEnvelope.cs

@@ -0,0 +1,15 @@
+using WebhookServer.Core.Models;
+
+namespace WebhookServer.Core.Callbacks;
+
+/// <summary>
+/// Internal queue item pairing a payload with the resolved <see cref="CallbackConfig"/>
+/// for the endpoint. The dispatcher reads from a channel of these.
+/// </summary>
+public sealed class CallbackEnvelope
+{
+    public required Guid EndpointId { get; init; }
+    public required string EndpointSlug { get; init; }
+    public required CallbackConfig Config { get; init; }
+    public required CallbackPayload Payload { get; init; }
+}