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/Models/DataPassingOptions.cs

@@ -0,0 +1,14 @@
+namespace WebhookServer.Core.Models;
+
+public sealed class DataPassingOptions
+{
+    public bool StdinJson { get; set; }
+    public bool EnvVars { get; set; }
+    public bool ArgTemplate { get; set; }
+
+    /// <summary>
+    /// Whitespace-separated list of template tokens; each rendered token becomes one argv entry.
+    /// Only used when <see cref="ArgTemplate"/> is true.
+    /// </summary>
+    public string? ArgTemplateString { get; set; }
+}