fg and bg builtins

[cdprice02]

Implement fg and bg builtins for job control.

Behavior:

• fg brings a background or stopped job to the foreground (shell waits for completion)
• bg resumes a stopped job in the background
• Both accept optional job number: fg %1
• Without argument, operate on the most recent job

Acceptance criteria:

• fg brings job to foreground and waits
• bg resumes stopped job in background
• Job number argument works (fg %1)
• Default to most recent job when no argument given
• Both registered as builtins