refactor: stand up vantage-ctl as its own repository
vantagectl/ becomes the repository root, so the module is gitea.hostxtra.co.uk/vantage/vantage-ctl and the image is vantage/vantage-ctl. The command keeps the name vantagectl. Release tags become a bare v*: the vantagectl/ prefix existed to namespace one component inside a shared repository, and unlike the agent's agent/v* nothing reads this one programmatically.
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