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/Storage/ConfigJson.cs

@@ -0,0 +1,27 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace WebhookServer.Core.Storage;
+
+/// <summary>
+/// Shared JSON serialization options used for persisting <see cref="Models.ServerConfig"/>
+/// and for IPC payloads. Keeps formatting and naming consistent.
+/// </summary>
+public static class ConfigJson
+{
+    public static readonly JsonSerializerOptions Pretty = new()
+    {
+        WriteIndented = true,
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+        Converters = { new JsonStringEnumConverter(JsonNamingPolicy.CamelCase) },
+    };
+
+    public static readonly JsonSerializerOptions Compact = new()
+    {
+        WriteIndented = false,
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+        Converters = { new JsonStringEnumConverter(JsonNamingPolicy.CamelCase) },
+    };
+}