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

@@ -975,6 +975,7 @@ function Sync-ClaudeSkill {
       throw "materializePaths source does not exist: $materializeSource"
     }
     Remove-PathIfExists -Path $materializeDestination -Within $RepoRoot
+    New-Item -ItemType Directory -Force -Path (Split-Path -Path $materializeDestination -Parent) | Out-Null
     Copy-Item -LiteralPath $materializeSource -Destination $materializeDestination -Recurse -Force
   }