fix(skills): frontmatter lint — using-interns v0.3.1 (YAML+≤1024), using-wiki-graph v0.1.1 (≤1024), ralph-loop-execution frontmatter, remove using-yt-tools stub, add scripts/lint-skills.py + wire into update.{sh,ps1}

scripts/update.ps1

@@ -133,6 +133,10 @@ if (Test-Path (Join-Path $internsMcp '.git')) {
 
 Pop-Location
 Write-Host "[update] Installing skills..." -ForegroundColor Cyan
+# 0. Lint skills against Agent Skills spec (fail on violations)
+& python "$root\scripts\lint-skills.py" "$root\skills" 2>$null
+if ($LASTEXITCODE -ne 0) { Write-Host "[update] SKILL.md lint failed - fix before install" -ForegroundColor Red; exit 1 }
+# 1. Install all skills via install.ps1
 & "$root\scripts\install.ps1"
 Write-Host "[ok] Skills installed." -ForegroundColor Green