fix: show helpful error when running git gtr cd (#137)

Author: helizaga

bin/git-gtr

@@ -106,6 +106,14 @@ main() {
     help|--help|-h)
       cmd_help "$@"
       ;;
+    cd)
+      local _shell_name
+      _shell_name="$(basename "${SHELL:-bash}")"
+      log_error "'cd' requires shell integration (subprocesses cannot change your shell's directory)"
+      log_info "Set up with: eval \"\$(git gtr init $_shell_name)\""
+      log_info "Then use:    gtr cd [<branch>]"
+      exit 1
+      ;;
     *)
       log_error "Unknown command: $cmd"
       echo "Use 'git gtr help' for available commands"