[Advanced] Implement Job Control (&, jobs, fg, bg)

[aphexlog]

Support running commands in the background and managing jobs.

Tasks:

• Implement & for background processes.
• Implement jobs to list background tasks.
• Implement fg and bg to resume jobs.

Acceptance Criteria:

• sleep 10 & runs in the background.
• jobs lists running jobs.
• fg brings a job back to the foreground.