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.

internal/updates/updates_windows.go

@@ -6,7 +6,7 @@ import (
 	"strings"
 	"time"
 
-	"gitea.hostxtra.co.uk/mrhid6/vantage/agent/internal/winexec"
+	"gitea.hostxtra.co.uk/vantage/vantage-agent/internal/winexec"
 )
 
 const (