fix(coding-agent): ignore SIGINT while process is suspended

[aliou]

When the user suspends pi with Ctrl+Z, pressing Ctrl+C in the terminal kills the backgrounded process because no SIGINT handler is active. This adds a no-op SIGINT handler before suspending and removes it on resume.

Change: packages/coding-agent/src/modes/interactive/interactive-mode.ts -- handleCtrlZ()

• Register a no-op SIGINT handler before sending SIGTSTP
• Remove it in the SIGCONT callback when the process resumes

[github-actions]

Hi @aliou, thanks for your interest in contributing!

We ask new contributors to open an issue first before submitting a PR. This helps us discuss the approach and avoid wasted effort.

Next steps:

1. Open an issue describing what you want to change and why (keep it concise, write in your human voice, AI slop will be closed)
2. Once a maintainer approves with lgtm, you'll be added to the approved contributors list
3. Then you can submit your PR

This PR will be closed automatically. See https://github.com/badlogic/pi-mono/blob/main/CONTRIBUTING.md for more details.

[aliou]

Thanks!