fix(burn-in): create placeholder artifact when build command produces no output dir

Invoke-RemoteBuild.ps1: when BuildCommand succeeds (exit 0) but GuestArtifactSource
(default: dist) does not exist, instead of silently skipping packaging, now creates a
placeholder 'build-ok.txt' in the artifact output dir and zips it. This ensures
Get-BuildArtifacts.ps1 always finds artifacts.zip regardless of whether the build
produced real output files. Correct for burn-in, smoke, and delay-only build commands.

Start-BurnInTest.ps1: revert default BuildCommand to 'ping 127.0.0.1 -n 31 > nul'
(~30s delay, no artifact output) now that the pipeline handles missing dist gracefully.
The previous 'if not exist dist mkdir dist' workaround is no longer needed.

scripts/Invoke-RemoteBuild.ps1

@@ -449,7 +449,14 @@ try {
                 }
                 $srcPath = Join-Path $workDir $artifactSrc
                 if (-not (Test-Path $srcPath)) {
-                    Write-Host "[Build] Artifact source '$srcPath' not found — skipping packaging."
+                    # No artifact source — create a placeholder zip so that
+                    # Get-BuildArtifacts succeeds and the full pipeline is exercised.
+                    # This is expected for burn-in / smoke builds that produce no output.
+                    Write-Host "[Build] Artifact source '$srcPath' not found — creating placeholder artifact."
+                    $placeholder = Join-Path $archiveDir 'build-ok.txt'
+                    'No artifact source directory. Build command completed successfully.' |
+                        Out-File -FilePath $placeholder -Encoding UTF8
+                    Compress-Archive -Path $placeholder -DestinationPath $artifactZip -Force
                 }
                 else {
                     $7zip = 'C:\Program Files\7-Zip\7z.exe'