[Security] Fix ineffective PID validation in treeKill

[gonzaloriestra]

The treeKill utility used Number.isNaN() on a string variable, which is ineffective for numeric validation as it returns false for non-numeric strings. This created a potential command injection risk on Windows where the PID is passed to exec().

This PR replaces the flawed check with a robust regex validation /^\d+$/ to ensure the PID is a valid numeric string.

How to test your changes?
Run the newly added tests:
pnpm --filter cli-kit vitest run tree-kill

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PR created automatically by Jules for task 2778250173359050246 started by @gonzaloriestra

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