feat(burn-in): add fake build script for burnin-dummy repo; update default BuildCommand

gitea/burnin-dummy/build.ps1:
  Fake build script to commit to the burnin-dummy repo.
  Creates dist\build-info.txt (date, host, pid) after ~30s ping delay.
  Invoke-RemoteBuild packages dist\ as artifacts.zip.

Start-BurnInTest.ps1:
  Default BuildCommand updated from ping-only delay to build.ps1 invocation.
  Run with -NoSkipArtifact to exercise the full artifact pipeline.

gitea/burnin-dummy/build.ps1

@@ -0,0 +1,33 @@
+#Requires -Version 5.1
+<#
+.SYNOPSIS
+    Fake build script per il repo burnin-dummy.
+
+.DESCRIPTION
+    Simula un build reale: attende ~30s (ping), poi scrive un file in dist\.
+    Invoke-RemoteBuild impacchetta dist\ come artifacts.zip al termine.
+    Committare questo file come build.ps1 nella root del repo burnin-dummy.
+#>
+
+Set-StrictMode -Version Latest
+$ErrorActionPreference = 'Stop'
+
+$distDir = Join-Path $PSScriptRoot 'dist'
+if (-not (Test-Path $distDir)) {
+    New-Item -ItemType Directory -Path $distDir | Out-Null
+}
+
+# Simula lavoro di build (~30 s).
+# ping e' l'unico delay affidabile in contesto cmd/WinRM (Start-Sleep non funziona via cmd /c).
+Write-Host '[build.ps1] Simulating build work (~30s)...'
+ping 127.0.0.1 -n 31 | Out-Null
+
+# Scrive un artifact marker con metadati del run.
+@"
+build-ok
+date=$(Get-Date -Format 'yyyy-MM-dd HH:mm:ss')
+host=$env:COMPUTERNAME
+pid=$PID
+"@ | Out-File -FilePath (Join-Path $distDir 'build-info.txt') -Encoding UTF8
+
+Write-Host "[build.ps1] Artifact written to $distDir"