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.

.gitignore

@@ -0,0 +1,4 @@
+dist
+.env
+*.tar.gz
+*.tar.gz.partial