fix: propagate SIGINT exit status on POSIX

[lawrence3699]

Fixes #470

When concurrently itself receives SIGINT on POSIX, it currently exits with code 0 after shutting down its children. That makes shell callers treat Ctrl+C as success, so && chains continue running.

This keeps the existing shutdown flow for child commands, but once the run finishes it re-sends SIGINT to the concurrently process on POSIX so the parent shell observes an interrupted command instead of a successful exit. Windows behavior stays unchanged.

Validation:

• pnpm build
• pnpm test -- --run bin/index.spec.ts -t 'propagates SIGINT on POSIX when interrupted'
• Reproduced the issue before and after with bash -lc 'node dist/bin/index.js "exec sleep 1000" "exec sleep 2000" && echo STILL_HERE' and a SIGINT sent to the process group: before the patch it printed STILL_HERE, after the patch it exited with SIGINT and did not print it

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