fix(docs): add explicit shutdown to quickstart example

[abhinav-galileo]

Summary

• update the README quickstart async example to call agent_control.ashutdown() in a finally block
• wrap the snippet with an if __name__ == "__main__": guard
• add a short note explaining that short-lived scripts should explicitly shut down so observability events flush before exit

Testing

• make check
• git push -u origin abhi/quickstart-explicit-shutdown (pre-push hook: lint, typecheck, contrib evaluator checks)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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