Sync third-party and MCP marketplace plugins

Constraint: Public skills are published only by explicit administrator action unless they are tracked third-party market sources.
Confidence: high
Scope-risk: narrow
Directive: Keep private/internal skills out of the public marketplace and preserve normal incremental market Git history.
Tested: Marketplace validation passed.

scripts/sync_external_plugins.ps1

@@ -990,6 +990,20 @@ function Sync-ClaudeSkill {
   Remove-ReadmeLines -Root $pluginDir -LineExcludes $Source.readmeLineExcludes
   Compress-ImagesToWebp -Root $skillDest -CompressPaths $Source.webpCompressPaths
 
+  if ($Source.PSObject.Properties.Name -contains "marketAssets" -and $Source.marketAssets) {
+    foreach ($asset in @($Source.marketAssets)) {
+      Assert-RelativePathValue -Path ([string]$asset.source)
+      Assert-RelativePathValue -Path ([string]$asset.destination)
+      $assetSource = Join-Path $RepoRoot ([string]$asset.source)
+      if (-not (Test-Path -LiteralPath $assetSource -PathType Leaf)) {
+        throw "marketAssets file does not exist: $assetSource"
+      }
+      $assetDestination = Join-Path $pluginDir ([string]$asset.destination)
+      New-Item -ItemType Directory -Force -Path (Split-Path -Path $assetDestination -Parent) | Out-Null
+      Copy-Item -LiteralPath $assetSource -Destination $assetDestination -Force
+    }
+  }
+
   $description = $metadata.description
   if (-not $description) {
     $description = $frontmatter["description"]