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

@@ -585,6 +585,60 @@ function Set-SkillFrontmatterValue {
   Write-Utf8NoBomText -Path $SkillFile -Text (($lines -join "`n") + "`n")
 }
 
+function Remove-SkillFrontmatterFields {
+  param(
+    [string]$SkillFile,
+    [object]$Fields
+  )
+  $fieldNames = @($Fields | ForEach-Object { ([string]$_).Trim() } | Where-Object { $_ })
+  if ($fieldNames.Count -eq 0) {
+    return
+  }
+  foreach ($fieldName in $fieldNames) {
+    if ($fieldName -notmatch '^[A-Za-z0-9_-]+$') {
+      throw "skillFrontmatterRemovals contains invalid field name: $fieldName"
+    }
+  }
+  $lines = @(Get-Content -LiteralPath $SkillFile)
+  if ($lines.Count -lt 3 -or $lines[0] -ne "---") {
+    return
+  }
+  $endIndex = -1
+  for ($i = 1; $i -lt $lines.Count; $i++) {
+    if ($lines[$i] -eq "---") {
+      $endIndex = $i
+      break
+    }
+  }
+  if ($endIndex -lt 0) {
+    return
+  }
+  $kept = @($lines[0])
+  $skipContinuation = $false
+  for ($i = 1; $i -lt $endIndex; $i++) {
+    $line = $lines[$i]
+    if ($skipContinuation) {
+      if (-not $line.Trim() -or $line -match '^\s') {
+        continue
+      }
+      $skipContinuation = $false
+    }
+    $remove = $false
+    foreach ($fieldName in $fieldNames) {
+      if ($line -match "^$([regex]::Escape($fieldName))\s*:") {
+        $remove = $true
+        $skipContinuation = $true
+        break
+      }
+    }
+    if (-not $remove) {
+      $kept += $line
+    }
+  }
+  $kept += $lines[$endIndex..($lines.Count - 1)]
+  Write-Utf8NoBomText -Path $SkillFile -Text (($kept -join "`n") + "`n")
+}
+
 function Apply-LocalOverrides {
   param(
     [string]$PluginDir,
@@ -611,6 +665,16 @@ function Apply-LocalOverrides {
       Set-SkillFrontmatterValue -SkillFile $skillFile -Key "description" -Value $property.Value
     }
   }
+  if ($Source.PSObject.Properties.Name -contains "skillFrontmatterRemovals" -and $Source.skillFrontmatterRemovals) {
+    $skillsRoot = Join-Path $PluginDir "skills"
+    foreach ($property in $Source.skillFrontmatterRemovals.PSObject.Properties) {
+      $skillFile = Join-Path (Join-Path $skillsRoot $property.Name) "SKILL.md"
+      if (-not (Test-Path -LiteralPath $skillFile)) {
+        throw "skillFrontmatterRemovals references missing skill: $skillFile"
+      }
+      Remove-SkillFrontmatterFields -SkillFile $skillFile -Fields $property.Value
+    }
+  }
 }
 
 function Get-Clone {