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/install-service.ps1

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+<#
+.SYNOPSIS
+    Installs and starts the WebhookServer Windows Service.
+
+.DESCRIPTION
+    Creates the service via sc.exe pointed at the published WebhookServer.Service.exe,
+    sets it to start automatically, and starts it. Re-running the script with the same
+    BinaryPath updates the binary path of an existing service.
+
+.PARAMETER BinaryPath
+    Full path to WebhookServer.Service.exe. Defaults to .\publish\WebhookServer.Service.exe
+    relative to the script.
+
+.PARAMETER ServiceAccount
+    Account to run the service under. Defaults to LocalSystem.
+    For Active-Directory-aware hooks pass a domain user (DOMAIN\user) or a gMSA
+    (DOMAIN\svc-name$ — note the trailing $). Domain users require -Password.
+    Never pass LocalService — it has no network identity and cannot reach a DC.
+
+.PARAMETER Password
+    Password for a domain-user account. Not required for LocalSystem, NetworkService,
+    LocalService, or gMSA accounts.
+
+.EXAMPLE
+    .\install-service.ps1 -BinaryPath C:\WebhookServer\WebhookServer.Service.exe
+
+.EXAMPLE
+    .\install-service.ps1 -BinaryPath C:\WebhookServer\WebhookServer.Service.exe `
+        -ServiceAccount 'CONTOSO\svc-webhookserver$'
+#>
+[CmdletBinding()]
+param(
+    [string]$BinaryPath = (Join-Path $PSScriptRoot '..\publish\WebhookServer.Service.exe'),
+    [string]$ServiceName = 'WebhookServer',
+    [string]$DisplayName = 'Webhook Server',
+    [string]$ServiceAccount = 'LocalSystem',
+    [string]$Password
+)
+
+$ErrorActionPreference = 'Stop'
+
+if ($ServiceAccount -ieq 'LocalService') {
+    throw 'LocalService has no network identity and cannot talk to a domain controller. Use LocalSystem, a domain user, or a gMSA instead.'
+}
+
+$BinaryPath = (Resolve-Path -LiteralPath $BinaryPath).Path
+if (-not (Test-Path -LiteralPath $BinaryPath)) {
+    throw "Binary not found: $BinaryPath"
+}
+
+# Build sc.exe argv. Note: sc.exe is fussy about spaces — keep "key= value" format.
+$obj = $ServiceAccount
+$existing = sc.exe query $ServiceName 2>$null
+
+if ($existing) {
+    Write-Host "Service '$ServiceName' already exists; updating binPath and account."
+    sc.exe config $ServiceName binPath= "`"$BinaryPath`"" obj= $obj $(if ($Password) { "password= $Password" }) | Out-Null
+} else {
+    $args = @(
+        'create', $ServiceName,
+        "binPath=", "`"$BinaryPath`"",
+        "DisplayName=", "`"$DisplayName`"",
+        "start=", "auto",
+        "obj=", $obj
+    )
+    if ($Password) { $args += @('password=', $Password) }
+    sc.exe @args | Out-Null
+}
+
+# Configure failure recovery: restart the service on first/second failure, reset count after a day.
+sc.exe failure $ServiceName reset= 86400 actions= restart/5000/restart/5000/restart/5000 | Out-Null
+
+Write-Host "Starting service '$ServiceName'..."
+sc.exe start $ServiceName | Out-Null
+
+Start-Sleep -Seconds 1
+sc.exe query $ServiceName

scripts/uninstall-service.ps1

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+<#
+.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.'