fix(burn-in): add -SkipArtifact switch; restore production fail-on-missing-dist

PROBLEM: Invoke-RemoteBuild.ps1 was creating a placeholder artifact when the build
produced no dist/ directory. In production this hides build defects (build exits 0
but produces nothing = silent failure treated as success).

SOLUTION: Add explicit -SkipArtifact switch to the call chain:

Invoke-RemoteBuild.ps1:
  - Add -SkipArtifact switch. When set: skips packaging entirely (no zip created).
  - When absent (production default): missing GuestArtifactSource is a hard error
    with a clear message pointing to -SkipArtifact as the opt-in for no-output builds.

Invoke-CIJob.ps1:
  - Add -SkipArtifact switch. Passes it to Invoke-RemoteBuild.ps1.
  - Phase 6 (Get-BuildArtifacts) is entirely skipped when set; logs 'skipped' event.

Test-CapacityBurnIn.ps1:
  - Add -SkipArtifact switch. Adds '-SkipArtifact' flag to each child job argList.

Start-BurnInTest.ps1:
  - Always passes -SkipArtifact by default (burn-in uses delay commands, no output).
  - Add -NoSkipArtifact override switch for runs that do produce real artifacts.
  - Remove -NoSkipArtifact escape from wrapper: default burn-in never needs artifacts.

scripts/Invoke-CIJob.ps1

@@ -105,6 +105,11 @@ param(
     # Requires Set-TemplateSharedFolders.ps1 to have been run on the template.
     [switch] $UseSharedCache,
 
+    # Skip artifact packaging and collection (Phase 5 packaging + Phase 6).
+    # Use for burn-in / smoke builds that intentionally produce no output.
+    # In production (absent) the build must produce artifacts or the job fails.
+    [switch] $SkipArtifact,
+
     # Credential Manager target for Gitea PAT used by -UseGitClone (private repos).
     # Store with: New-StoredCredential -Target 'GiteaPAT' -UserName '<user>' -Password '<pat>' -Persist LocalMachine
     # Ignored when -UseGitClone is not set.
@@ -494,35 +499,40 @@ try {
     if ($GuestArtifactSource -ne '') { $remoteBuildParams['GuestArtifactSource'] = $GuestArtifactSource }
     if ($ExtraGuestEnv.Count -gt 0)  { $remoteBuildParams['ExtraGuestEnv']       = $ExtraGuestEnv }
     if ($UseSharedCache)             { $remoteBuildParams['UseSharedCache']       = $true }
+    if ($SkipArtifact)               { $remoteBuildParams['SkipArtifact']         = $true }
     & "$scriptDir\Invoke-RemoteBuild.ps1" @remoteBuildParams
     Write-JobEvent -Phase 'phase5.build' -Status 'success'
     $phaseTimings.Add([PSCustomObject]@{ Phase = 'Phase 5 - Remote build'; Sec = [int]((Get-Date) - $phaseStart).TotalSeconds })
     $phaseStart = Get-Date
 
     # ── Phase 6: Collect artifacts ────────────────────────────────────────
-    Write-Host "`n[Phase 6/6] Collecting artifacts..."
-    Write-JobEvent -Phase 'phase6.artifacts' -Status 'start'
-    if ($GuestOS -eq 'Linux') {
-        & "$scriptDir\Get-BuildArtifacts.ps1" `
-            -IPAddress         $VMIPAddress `
-            -GuestOS           'Linux' `
-            -SshKeyPath        $SshKeyPath `
-            -SshUser           $SshUser `
-            -SshKnownHostsFile $SshKnownHostsFile `
-            -GuestArtifactPath '/opt/ci/output' `
-            -HostArtifactDir   $jobArtifactDir `
-            -IncludeLogs
+    if ($SkipArtifact) {
+        Write-Host "`n[Phase 6/6] Artifact collection skipped (-SkipArtifact)."
+        Write-JobEvent -Phase 'phase6.artifacts' -Status 'skipped'
     } else {
-        & "$scriptDir\Get-BuildArtifacts.ps1" `
-            -IPAddress         $VMIPAddress `
-            -Credential        $credential `
-            -GuestArtifactPath 'C:\CI\output\artifacts.zip' `
-            -HostArtifactDir   $jobArtifactDir `
-            -IncludeLogs
+        Write-Host "`n[Phase 6/6] Collecting artifacts..."
+        Write-JobEvent -Phase 'phase6.artifacts' -Status 'start'
+        if ($GuestOS -eq 'Linux') {
+            & "$scriptDir\Get-BuildArtifacts.ps1" `
+                -IPAddress         $VMIPAddress `
+                -GuestOS           'Linux' `
+                -SshKeyPath        $SshKeyPath `
+                -SshUser           $SshUser `
+                -SshKnownHostsFile $SshKnownHostsFile `
+                -GuestArtifactPath '/opt/ci/output' `
+                -HostArtifactDir   $jobArtifactDir `
+                -IncludeLogs
+        } else {
+            & "$scriptDir\Get-BuildArtifacts.ps1" `
+                -IPAddress         $VMIPAddress `
+                -Credential        $credential `
+                -GuestArtifactPath 'C:\CI\output\artifacts.zip' `
+                -HostArtifactDir   $jobArtifactDir `
+                -IncludeLogs
+        }
+        Write-JobEvent -Phase 'phase6.artifacts' -Status 'success' -Data @{ dir = $jobArtifactDir }
     }
-
     $elapsed = (Get-Date) - $startTime
-    Write-JobEvent -Phase 'phase6.artifacts' -Status 'success' -Data @{ dir = $jobArtifactDir }
     Write-JobEvent -Phase 'job' -Status 'success' -Data @{ elapsedSec = [int]$elapsed.TotalSeconds }
     $phaseTimings.Add([PSCustomObject]@{ Phase = 'Phase 6 - Artifacts'; Sec = [int]((Get-Date) - $phaseStart).TotalSeconds })
     Write-Host "`n============================================================"