feat: Windows Service Wrapper für NexusRMM Agent

- winsvc package mit Build-Tag-basierter plattformspezifischer Implementierung:
  - service_windows.go: svc.Handler + Install/Uninstall/Start/Stop via windows/svc/mgr
  - service_stub.go: Stub-Implementierungen für nicht-Windows Builds
- main.go refaktoriert:
  - os.Executable() für absoluten Config-Pfad (Service-Modus: CWD = C:\Windows\System32)
  - Kommandozeilen-Args: install / uninstall / start / stop
  - winsvc.IsWindowsService() Erkennung → Service-Modus oder Konsolen-Modus
  - Agent-Loop als makeRunFn() extrahiert (wiederverwendbar für beide Modi)
- install-service.ps1: Convenience-Skript zum Bauen, Installieren und Starten

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

install-service.ps1

@@ -0,0 +1,127 @@
+#Requires -RunAsAdministrator
+<#
+.SYNOPSIS
+    Installiert den NexusRMM Agent als Windows Service
+.DESCRIPTION
+    Baut den Agent, kopiert ihn ins Zielverzeichnis und registriert ihn als Windows Service.
+    Muss als Administrator ausgefuehrt werden.
+.PARAMETER ServerAddress
+    gRPC-Adresse des NexusRMM Servers (Standard: localhost:5001)
+.PARAMETER InstallDir
+    Installationsverzeichnis (Standard: C:\Program Files\NexusRMM)
+#>
+param(
+    [string]$ServerAddress = "localhost:5001",
+    [string]$InstallDir = "C:\Program Files\NexusRMM"
+)
+
+$ErrorActionPreference = "Continue"
+$Root = $PSScriptRoot
+
+function Write-Step { param($msg) Write-Host "" ; Write-Host "==> $msg" -ForegroundColor Cyan }
+function Write-OK   { param($msg) Write-Host "    [OK] $msg"     -ForegroundColor Green }
+function Write-Fail { param($msg) Write-Host "    [FEHLER] $msg" -ForegroundColor Red }
+
+# ---------------------------------------------------------------------------
+# 1. Agent bauen
+# ---------------------------------------------------------------------------
+Write-Step "Baue NexusRMM Agent..."
+
+Set-Location "$Root\Agent"
+$env:GOOS = "windows"
+$env:GOARCH = "amd64"
+go build -ldflags "-s -w -X main.version=1.0.0" -o nexus-agent.exe ./cmd/agent
+if ($LASTEXITCODE -ne 0) {
+    Write-Fail "go build fehlgeschlagen."
+    exit 1
+}
+Write-OK "nexus-agent.exe erstellt."
+
+# ---------------------------------------------------------------------------
+# 2. Installationsverzeichnis anlegen
+# ---------------------------------------------------------------------------
+Write-Step "Lege Installationsverzeichnis an: $InstallDir"
+
+if (-not (Test-Path $InstallDir)) {
+    New-Item -ItemType Directory -Path $InstallDir -Force | Out-Null
+}
+Write-OK "Verzeichnis bereit."
+
+# ---------------------------------------------------------------------------
+# 3. Dateien kopieren
+# ---------------------------------------------------------------------------
+Write-Step "Kopiere Dateien..."
+
+Copy-Item "$Root\Agent\nexus-agent.exe" "$InstallDir\nexus-agent.exe" -Force
+
+# Konfigurationsdatei anlegen falls nicht vorhanden
+$configPath = "$InstallDir\nexus-agent.yaml"
+if (-not (Test-Path $configPath)) {
+    @"
+serverAddress: $ServerAddress
+agentId: ""
+heartbeatInterval: 30
+meshEnabled: false
+meshCentralUrl: ""
+"@ | Out-File -FilePath $configPath -Encoding utf8
+    Write-OK "nexus-agent.yaml erstellt (serverAddress: $ServerAddress)."
+} else {
+    Write-OK "nexus-agent.yaml bereits vorhanden, wird nicht ueberschrieben."
+}
+
+# ---------------------------------------------------------------------------
+# 4. Bestehenden Service stoppen/deinstallieren falls vorhanden
+# ---------------------------------------------------------------------------
+$exePath = "$InstallDir\nexus-agent.exe"
+
+$svcCheck = Get-Service -Name "NexusRMMAgent" -ErrorAction SilentlyContinue
+if ($svcCheck) {
+    Write-Step "Entferne vorhandenen Service..."
+    & $exePath stop 2>$null
+    Start-Sleep -Seconds 2
+    & $exePath uninstall
+    if ($LASTEXITCODE -ne 0) {
+        Write-Fail "Service-Deinstallation fehlgeschlagen."
+        exit 1
+    }
+    Write-OK "Alter Service entfernt."
+}
+
+# ---------------------------------------------------------------------------
+# 5. Service installieren und starten
+# ---------------------------------------------------------------------------
+Write-Step "Installiere Service..."
+
+& $exePath install
+if ($LASTEXITCODE -ne 0) {
+    Write-Fail "Service-Installation fehlgeschlagen."
+    exit 1
+}
+Write-OK "Service installiert."
+
+Write-Step "Starte Service..."
+& $exePath start
+if ($LASTEXITCODE -ne 0) {
+    Write-Fail "Service konnte nicht gestartet werden."
+    exit 1
+}
+Write-OK "Service gestartet."
+
+# ---------------------------------------------------------------------------
+# Zusammenfassung
+# ---------------------------------------------------------------------------
+Write-Host ""
+Write-Host "+--------------------------------------------------+" -ForegroundColor Green
+Write-Host "|   NexusRMM Agent als Service installiert!       |" -ForegroundColor Green
+Write-Host "+--------------------------------------------------+" -ForegroundColor Green
+Write-Host "|  Installiert in: $InstallDir" -ForegroundColor Green
+Write-Host "|  Server:         $ServerAddress" -ForegroundColor Green
+Write-Host "|  Service-Name:   NexusRMMAgent" -ForegroundColor Green
+Write-Host "+--------------------------------------------------+" -ForegroundColor Green
+Write-Host ""
+Write-Host "Verwalten:" -ForegroundColor Yellow
+Write-Host "  Start:       & '$exePath' start" -ForegroundColor Yellow
+Write-Host "  Stop:        & '$exePath' stop" -ForegroundColor Yellow
+Write-Host "  Entfernen:   & '$exePath' uninstall" -ForegroundColor Yellow
+Write-Host "  Oder via:    services.msc -> NexusRMMAgent" -ForegroundColor Yellow
+Write-Host ""