docs: add --headed flag to SKILL.md open parameters

Author: vivainio

skills/playwright-cli/SKILL.md

@@ -157,6 +157,9 @@ playwright-cli video-stop video.webm
 
 ## Open parameters
 ```bash
+# Open with visible browser window (headless by default)
+playwright-cli open --headed
+
 # Use specific browser when creating session
 playwright-cli open --browser=chrome
 playwright-cli open --browser=firefox