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

@@ -871,6 +871,27 @@ function Sync-SkillCollection {
   $skillDirs = @(Get-ChildItem -LiteralPath $sourceSkillsRoot -Directory | Where-Object {
       Test-Path -LiteralPath (Join-Path $_.FullName "SKILL.md")
     } | Sort-Object Name)
+  $includeSkills = @()
+  if ($Source.PSObject.Properties.Name -contains "includeSkills" -and $Source.includeSkills) {
+    $includeSkills = @($Source.includeSkills | ForEach-Object { ([string]$_).Trim() } | Where-Object { $_ })
+  }
+  if ($includeSkills.Count -gt 0) {
+    $skillDirsByName = @{}
+    foreach ($skillDir in $skillDirs) {
+      $skillDirsByName[$skillDir.Name] = $skillDir
+    }
+    $selectedSkillDirs = @()
+    foreach ($skillName in $includeSkills) {
+      if ($skillName -notmatch '^[a-z0-9]+(?:-[a-z0-9]+)*$') {
+        throw "skill-collection includeSkills contains invalid name: $skillName"
+      }
+      if (-not $skillDirsByName.ContainsKey($skillName)) {
+        throw "skill-collection includeSkills skill not found: $skillName"
+      }
+      $selectedSkillDirs += $skillDirsByName[$skillName]
+    }
+    $skillDirs = $selectedSkillDirs
+  }
   if ($skillDirs.Count -eq 0) {
     throw "skill-collection adapter found no child SKILL.md files: $sourceSkillsRoot"
   }