fix(build): reject unknown tokens beside allowed placeholders

scripts/test-build-validation.ps1

@@ -24,6 +24,9 @@ function Test-BuildFails([string]$Label, [scriptblock]$Mutate) {
     if (-not $failed) { throw "Expected build to fail for case '$Label' but it succeeded." }
 }
 
+$baseline = New-RepoCopy
+& (Join-Path $baseline 'scripts/build.ps1') -Summary | Out-Null
+
 Test-BuildFails 'invalid Claude settings.json' {
     param($copy)
     Add-Content (Join-Path $copy 'adapters/claude/config/settings.json') '} this is not json {'
@@ -47,6 +50,12 @@ Test-BuildFails 'leftover template placeholder' {
     Add-Content (Join-Path $copy 'shared/global-instructions.md') "`n__NOT_A_REAL_PLACEHOLDER__"
 }
 
+Test-BuildFails 'unknown placeholder beside an allowed installer placeholder' {
+    param($copy)
+    $path = Join-Path $copy 'adapters/claude/config/settings.json'
+    (Get-Content $path -Raw).Replace('__CLAUDE_CODE_MAX_TOOL_USE_CONCURRENCY__', '__CLAUDE_CODE_MAX_TOOL_USE_CONCURRENCY____UNKNOWN_PLACEHOLDER__') | Set-Content $path
+}
+
 Remove-Item $work -Recurse -Force -ErrorAction SilentlyContinue
 if ($Summary) { Write-Output 'Tests: PASS | build validation' } else { Write-Output 'PASS build validation: invalid JSON, duplicate agent, duplicate plugin, and leftover placeholders are all rejected' }
 exit 0