Fix stale warmup status persisting after rules are removed from config

spawn_warmup() returned early when rules were empty without clearing
status.warmup, leaving orphaned entries visible in web UI and CLI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/supervisor.rs

@@ -229,6 +229,9 @@ fn spawn_warmup(
     shutdown: &Arc<AtomicBool>,
 ) {
     if config.warmup.rules.is_empty() || !config.warmup.auto {
+        // Clear stale warmup status when rules are removed or auto is disabled
+        let mut status = shared_status.write().unwrap();
+        status.warmup.clear();
         return;
     }