PB-122: handle sigint properly

[little-dude]

References

https://xainag.atlassian.net/browse/PB-122

Summary

Guard the call to main() at the end of __main__.py. Otherwise
we're starting the coordinator twice by calling the coordinator
command: once when __main__.py is imported, and once when the
entrypoint (__main.py__::main) is called.

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Reviewer checklist

Reviewer agreement:

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Merge request checklist

• Conforms to the merge request title naming XP-XXX <a description in imperative form>.
• Each commit conforms to the naming convention XP-XXX <a description in imperative form>.
• Linked the ticket in the merge request title or the references section.
• Added an informative merge request summary.

Code checklist

• Conforms to the branch naming XP-XXX-<a_small_stub>.
• Passed scope checks.
• Added or updated tests if needed.
• Added or updated code documentation if needed.
• Conforms to Google docstring style.
• Conforms to XAIN structlog style.