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/ExecutionContext.cs

@@ -0,0 +1,18 @@
+using System.Text.Json.Nodes;
+
+namespace WebhookServer.Core.Execution;
+
+/// <summary>
+/// All data the executor needs from the inbound HTTP request.
+/// </summary>
+public sealed class ExecutionContext
+{
+    public required string RunId { get; init; }
+    public required string Slug { get; init; }
+    public required byte[] BodyBytes { get; init; }
+    public required string BodyString { get; init; }
+    public JsonNode? BodyJson { get; init; }
+    public required IReadOnlyDictionary<string, string> Headers { get; init; }
+    public required IReadOnlyDictionary<string, string> Query { get; init; }
+    public required IReadOnlyDictionary<string, string> Route { get; init; }
+}