app: add loopback multi-project registry

internal/mcp/mcp.go

@@ -188,6 +188,9 @@ func RunStdioProxy(socket, identity string) error {
 	// "<token>"} + newline. Real protocol handshake is a later
 	// milestone.
 	greeting := map[string]string{"patterm_identity": identity}
+	if key := os.Getenv("PATTERM_PROJECT_KEY"); key != "" {
+		greeting["project_key"] = key
+	}
 	gb, _ := json.Marshal(greeting)
 	gb = append(gb, '\n')
 	if _, err := conn.Write(gb); err != nil {