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>

scripts/uninstall-service.ps1

@@ -0,0 +1,38 @@
+<#
+.SYNOPSIS
+    Stops and removes the WebhookServer Windows Service.
+
+.DESCRIPTION
+    Leaves C:\ProgramData\WebhookServer (config + logs) untouched. Pass -PurgeData
+    to remove that directory as well.
+#>
+[CmdletBinding()]
+param(
+    [string]$ServiceName = 'WebhookServer',
+    [switch]$PurgeData
+)
+
+$ErrorActionPreference = 'Stop'
+
+$existing = sc.exe query $ServiceName 2>$null
+if (-not $existing) {
+    Write-Host "Service '$ServiceName' is not installed."
+    return
+}
+
+Write-Host "Stopping service '$ServiceName'..."
+sc.exe stop $ServiceName 2>$null | Out-Null
+Start-Sleep -Seconds 2
+
+Write-Host "Deleting service '$ServiceName'..."
+sc.exe delete $ServiceName | Out-Null
+
+if ($PurgeData) {
+    $dataRoot = Join-Path $env:ProgramData 'WebhookServer'
+    if (Test-Path -LiteralPath $dataRoot) {
+        Write-Host "Removing $dataRoot"
+        Remove-Item -LiteralPath $dataRoot -Recurse -Force
+    }
+}
+
+Write-Host 'Done.'