Signal handling (SIGINT, SIGTSTP, SIGCHLD)

[cdprice02]

Handle Unix signals correctly for a robust interactive shell.

Behavior:

• SIGINT (Ctrl+C): forward to foreground process group, don't kill the shell
• SIGTSTP (Ctrl+Z): suspend foreground process, return control to shell
• SIGCHLD: reap completed background jobs
• Provide clear feedback (e.g., print newline after Ctrl+C, don't leave prompt broken)

Per ferrish's explicit over implicit principle, signal behavior should be predictable and visible.

Platform note: This is primarily Unix/macOS. On Windows, consider graceful no-ops or platform-specific alternatives.

Acceptance criteria:

• Ctrl+C doesn't kill the shell
• Ctrl+Z suspends foreground process
• Background jobs are reaped via SIGCHLD
• User gets clear feedback on signal delivery
• Windows builds compile cleanly (graceful degradation)