Avoid shell execution in kill

[Dremig]

Summary

This updates kill to avoid constructing shell commands from pid values.

For valid input, the behavior stays the same, but the implementation now:

• normalizes and validates pid as a positive integer
• uses process.kill(pid, 'SIGKILL') on Unix-like platforms
• uses spawnSync('taskkill', [...]) on Windows instead of shell execution

This makes the helper safer and avoids shell interpretation of invalid values.

Tests

• node bin/licia.js format kill
• node bin/licia.js lint kill
• env HOME=/private/tmp/licia-home node bin/licia.js test kill -s