refactor: stand up vantage-agent as its own repository

agent/ becomes the repository root, so the module is
gitea.hostxtra.co.uk/vantage/vantage-agent, and installer/ comes with it.

Releases move to this repository, and internal/sync's self-update URLs
move with them. Agents built before this point have the old
mrhid6/vantage path compiled in and will fail the push-button update;
re-running the server-generated /update one-liner on each host moves
them onto a build that knows the new address.

.gitignore

@@ -0,0 +1,5 @@
+dist
+installer/vantage-agent-windows-amd64.exe
+installer/vantage-agent.msi
+installer/checksums-msi.txt
+.env