docs: require browser cleanup after agent tasks

[jackwener]

Summary

• diagnose the leftover-tab report as a browser-skill lifecycle issue rather than a site-adapter bug
• make opencli browser close an explicit required final step for agent-driven search / browse / debug tasks
• clarify that the browser window may stay open during the task, but should be closed unless the user asked to keep it open

Testing

• not run (skill documentation change only)