fix(commands): surface core fetch errors, run post-install in the project, exit 1 on invalid project command; document compat flow

packages/commands/duckbrain

@@ -28,7 +28,8 @@ if ($command === null || in_array($command, $toolboxCommands, true)) {
 $coreCommand = 'Commands\\' . ucfirst($command);
 
 if (!class_exists($coreCommand) && !file_exists(ROOT_CORE . '/Commands/' . ucfirst($command) . '.php')) {
-    exit("Invalid command.\n");
+    fwrite(STDERR, "Invalid command.\n");
+    exit(1);
 }
 
 $cli = new $coreCommand();
@@ -36,5 +37,6 @@ $cli = new $coreCommand();
 if ($cli instanceof Interfaces\CommandInterface) {
     call_user_func_array([$cli, 'run'], array_slice($argv, 2));
 } else {
-    exit("Invalid command.\n");
+    fwrite(STDERR, "La clase $coreCommand no implementa CommandInterface.\n");
+    exit(1);
 }