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.

Dockerfile

@@ -1,10 +1,13 @@
 # Build stage
 #
-# Context is vantagectl/ itself. It used to be the repository root, so that
-# shared/ could be copied in beside it; shared is now the private module
-# gitea.hostxtra.co.uk/vantage/vantage-shared, fetched like any other
-# dependency. The credential for it arrives as a BuildKit secret rather than a
-# build arg, which would be baked into this stage's layer history.
+# Context is the repository root.
+#
+# The command stays named vantagectl even though the repository is vantage-ctl:
+# it is what an operator types, and it is in every runbook. Only the paths moved.
+#
+# vantage-shared is a private module, so both steps below need a credential. It
+# arrives as a BuildKit secret rather than a build arg, which would be baked
+# into this stage's layer history.
 FROM golang:1.26 AS builder
 
 WORKDIR /src