Adds a simple .sh script that can trigger a refetch locally from a host
if required.
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## Summary by CodeRabbit
* **Documentation**
* Updated the Linux script used to trigger Fleet refetch operations. It
now runs with stricter Bash safety, validates and trims the system
identifier before sending the request, and reports clear success/failure
status while returning the appropriate nonzero exit code on errors.
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Gray Williams
d7800eb5c5