Support background execution

When a process is executed in the background, it stops when reading from the input stream.
Because of the `readInputLoop`, the `mvnd` process is stopped immediately.
This involves checking the SIGTSTP with JLine, stopping the thread and resuming it when the SIGCONT is received.

Start by locating the readInputLoop implementation and its JLine input handling. Trace how SIGTSTP and SIGCONT are currently handled, then verify that background execution no longer stops mvnd and that input processing resumes after SIGCONT. No specific test or file is named in the issue.

Written by the indexing model from the issue text.