fix: reject parent directory template locators

[immanuwell]

It fixes tiny edge case

base: .. slipped past the parent-dir check because it only looked for ../. That lets limactl template copy absolutize it to the parent of the template dir.

Repro on main:

tmpdir=$(mktemp -d)
printf "base: ..\n" > "$tmpdir/template.yaml"
limactl template copy "$tmpdir/template.yaml" -

This prints a parent path instead of rejecting it. now it errors, same vibe as ../base.yaml.

Tests:

go test ./...

[jandubois]

Thanks, this is a good improvement. Please also update the error message to match:

Suggested change

	return "", fmt.Errorf("relative locator path %q must not contain '../' segments", locator)
	return "", fmt.Errorf("relative locator path %q must not contain '..' path segments", locator)