docs(skill): require agent session cleanup

[Danil Pismenny (dapi)]

Summary

• make browser-session cleanup an explicit agent lifecycle requirement
• use a named session throughout the quick-start workflow
• document failure-safe cleanup and protect concurrent agents from global close/kill commands

This mitigates #460 for normal agent completion. It does not replace a daemon-level lifetime bound for crashes or forcibly terminated agents.

Validation

• skill-creator quick validation passes
• \ passes

[Danil Pismenny (dapi)]

Closing this upstream proposal; we will maintain the system-wide agent lifecycle policy in our fork.