Rename _diag -> diag (underscore folders are private in App Router)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude/scheduled_tasks.lock

@@ -0,0 +1 @@
+{"sessionId":"33150643-d635-40ed-aa16-be7e64c5285d","pid":54935,"procStart":"Mon Jun  1 23:45:35 2026","acquiredAt":1780364768253}

app/api/{_diag → diag}/route.ts

@@ -10,7 +10,7 @@ export const dynamic = "force-dynamic"
 // Space hardware to isolate which column / read-path / thread setting
 // triggers it. Remove once the root cause is fixed.
 //
-// Usage: /api/_diag?id=deepseek/deepseek-v4-flash[&threads=1][&http=1]
+// Usage: /api/diag?id=deepseek/deepseek-v4-flash[&threads=1][&http=1]
 
 const NESTED_COLUMNS = [
   "model_info",