Support non-global label prefixes in Confluence label cmdlets #20

During review of #17, Copilot suggested that Remove-ConfluenceLabel add a -Prefix parameter (defaulting to global) and include it in the DELETE query.

The current behavior is correct for the common case: the v1 DELETE /wiki/rest/api/content/{id}/label?name=<name> endpoint defaults prefix to global, so removing a global label by name works. This is proven by the passing integration test in #17, which removes a label by name and then asserts it is no longer present on the page.

This issue tracks the optional enhancement of supporting non-global label prefixes (my, team, system) across the label cmdlets (Add-ConfluenceLabel, Get-ConfluenceLabel, Remove-ConfluenceLabel) by adding an optional -Prefix parameter, so labels with a non-default prefix can be managed. This is out of scope for #17, which delivers global-label management.