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/CallbackPayload.cs

@@ -0,0 +1,22 @@
+using System.Text.Json.Serialization;
+
+namespace WebhookServer.Core.Callbacks;
+
+/// <summary>
+/// JSON body POSTed to a configured outbound callback URL.
+/// </summary>
+public sealed class CallbackPayload
+{
+    [JsonPropertyName("runId")] public required string RunId { get; init; }
+    [JsonPropertyName("endpoint")] public required string Endpoint { get; init; }
+    [JsonPropertyName("startedAt")] public required DateTimeOffset StartedAt { get; init; }
+    [JsonPropertyName("completedAt")] public required DateTimeOffset CompletedAt { get; init; }
+    [JsonPropertyName("durationMs")] public required long DurationMs { get; init; }
+    [JsonPropertyName("exitCode")] public required int ExitCode { get; init; }
+    [JsonPropertyName("succeeded")] public required bool Succeeded { get; init; }
+    [JsonPropertyName("timedOut")] public required bool TimedOut { get; init; }
+    [JsonPropertyName("stdout")] public string? Stdout { get; init; }
+    [JsonPropertyName("stderr")] public string? Stderr { get; init; }
+    [JsonPropertyName("stdoutTruncated")] public bool StdoutTruncated { get; init; }
+    [JsonPropertyName("stderrTruncated")] public bool StderrTruncated { get; init; }
+}