fix(burn-in): default BuildCommand creates artifact dir to exercise full pipeline

'exit 0' left C:\CI\build\dist non-existent -> packaging skipped -> Get-BuildArtifacts
could not find artifacts.zip -> all 4 jobs FAIL at artifact collection phase.

New default: 'if not exist dist mkdir dist & echo burnin_ok > dist\marker.txt'
This creates dist\marker.txt relative to C:\CI\build (GuestWorkDir), so the
packaging step finds it, creates artifacts.zip, and Get-BuildArtifacts succeeds.

Also updated docblock: clarify command runs via cmd /c; replace Start-Sleep example
(PS cmdlet, does not work via WinRM cmd) with ping delay equivalent.

scripts/Start-BurnInTest.ps1

@@ -33,9 +33,11 @@
     Job concorrenti per round. Default: 4.
 
 .PARAMETER BuildCommand
-    Comando da eseguire nel guest come passo build.
-    Default: 'exit 0' (smoke veloce — nessun build reale).
-    Usare 'Start-Sleep 120' per mantenere 4 VM attive e osservare la concorrenza.
+    Comando da eseguire nel guest come passo build (eseguito via cmd /c nel guest).
+    Default: crea la directory dist con un file marker, poi esce 0 — esercita
+    l'intero pipeline inclusa la fase di packaging e artifact collection.
+    Usare 'ping 127.0.0.1 -n 121 > nul' per mantenere 4 VM attive ~120s
+    e osservare la concorrenza senza produrre artifact.
 
 .PARAMETER RoundTimeoutMinutes
     Minuti massimi di attesa per ogni round prima di interrompere i job rimasti.
@@ -44,7 +46,7 @@
 param(
     [int]    $Rounds              = 3,
     [int]    $Parallelism         = 4,
-    [string] $BuildCommand        = 'exit 0',
+    [string] $BuildCommand        = 'if not exist dist mkdir dist & echo burnin_ok > dist\marker.txt',
     [int]    $RoundTimeoutMinutes = 60
 )