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/ArgTemplateRenderer.cs

@@ -0,0 +1,116 @@
+using System.Text.Json.Nodes;
+
+namespace WebhookServer.Core.Execution;
+
+/// <summary>
+/// Resolves {{path}} tokens against an <see cref="ExecutionContext"/>. Each whitespace-
+/// separated token in the template becomes one argv entry.
+/// Path grammar:
+///   {{body.foo.bar}}      JSON path into the body
+///   {{header.X-Foo}}      header by name (case-insensitive)
+///   {{query.bar}}         query param
+///   {{route.slug}}        route value
+/// Missing paths render as empty string.
+/// </summary>
+public static class ArgTemplateRenderer
+{
+    public static List<string> Render(string? template, ExecutionContext ctx)
+    {
+        var args = new List<string>();
+        if (string.IsNullOrWhiteSpace(template)) return args;
+
+        foreach (var token in template.Split((char[]?)null, StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries))
+        {
+            args.Add(RenderToken(token, ctx));
+        }
+        return args;
+    }
+
+    private static string RenderToken(string token, ExecutionContext ctx)
+    {
+        // Replace every {{...}} occurrence inside the token in a single left-to-right pass.
+        var result = new System.Text.StringBuilder(token.Length);
+        var i = 0;
+        while (i < token.Length)
+        {
+            var open = token.IndexOf("{{", i, StringComparison.Ordinal);
+            if (open < 0)
+            {
+                result.Append(token, i, token.Length - i);
+                break;
+            }
+            result.Append(token, i, open - i);
+            var close = token.IndexOf("}}", open + 2, StringComparison.Ordinal);
+            if (close < 0)
+            {
+                // Unclosed token — treat the rest as literal.
+                result.Append(token, open, token.Length - open);
+                break;
+            }
+            var path = token.Substring(open + 2, close - (open + 2)).Trim();
+            result.Append(Resolve(path, ctx));
+            i = close + 2;
+        }
+        return result.ToString();
+    }
+
+    private static string Resolve(string path, ExecutionContext ctx)
+    {
+        if (string.IsNullOrEmpty(path)) return "";
+        var dot = path.IndexOf('.');
+        if (dot < 0) return "";
+
+        var scope = path[..dot];
+        var rest = path[(dot + 1)..];
+
+        return scope.ToLowerInvariant() switch
+        {
+            "body" => ResolveJson(ctx.BodyJson, rest),
+            "header" => LookupCaseInsensitive(ctx.Headers, rest),
+            "query" => LookupCaseInsensitive(ctx.Query, rest),
+            "route" => LookupCaseInsensitive(ctx.Route, rest),
+            _ => "",
+        };
+    }
+
+    private static string ResolveJson(JsonNode? root, string path)
+    {
+        if (root is null) return "";
+        JsonNode? cursor = root;
+        foreach (var segment in path.Split('.'))
+        {
+            if (cursor is null) return "";
+
+            if (cursor is JsonObject obj)
+            {
+                cursor = obj.TryGetPropertyValue(segment, out var next) ? next : null;
+                continue;
+            }
+
+            if (cursor is JsonArray arr && int.TryParse(segment, out var idx))
+            {
+                cursor = idx >= 0 && idx < arr.Count ? arr[idx] : null;
+                continue;
+            }
+
+            return "";
+        }
+
+        return cursor switch
+        {
+            null => "",
+            JsonValue v => v.ToString(),
+            _ => cursor.ToJsonString(),
+        };
+    }
+
+    private static string LookupCaseInsensitive(IReadOnlyDictionary<string, string> map, string key)
+    {
+        foreach (var kvp in map)
+        {
+            if (string.Equals(kvp.Key, key, StringComparison.OrdinalIgnoreCase))
+                return kvp.Value;
+        }
+        return "";
+    }
+}