refactor: move the agent and its installer to vantage-agent

agent/ becomes the root of gitea.hostxtra.co.uk/vantage/vantage-agent,
with installer/ alongside it, and agent-release.yml goes with them.

Releases now come from that repository, so the six places this server
generates or reads a release URL are repointed: both install scripts,
both update scripts, and the latest-version lookup in dispatch.go. The
agent/v* tag prefix is unchanged — those scripts grep for it.

Agents built before this move have the old mrhid6/vantage path compiled
into their self-update and will 404 on the push-button update. The
remedy is the /update one-liner, which this server generates and which
therefore has to ship first.

server/internal/services/dispatch.go

@@ -317,7 +317,7 @@ type KeyGenParams struct {
 
 func GetLatestAgentVersion() (string, error) {
 	giteaHost := "gitea.hostxtra.co.uk"
-	url := fmt.Sprintf("https://%s/api/v1/repos/mrhid6/vantage/releases?limit=20", giteaHost)
+	url := fmt.Sprintf("https://%s/api/v1/repos/vantage/vantage-agent/releases?limit=20", giteaHost)
 	resp, err := http.Get(url)
 	if err != nil {
 		return "", fmt.Errorf("fetch releases: %w", err)