phpcheck-url: Check PHP version from URL

From this skill's directory, run: ```sh node scripts/phpcheck.js "https://example.com" --json ``` Use HTTPS for a bare hostname. The checker reads `Server` and `X-Powered-By` headers. If the version is hidden, report unknown. Versions below an upstream fix return `potentially_vulnerable` with exit code 1. PHP 5.4–5.6, 7.0–7.4, 8.0 and 8.1 also return this warning. A higher patch number on those branches, such as 8.1.35, does not establish a fix. Exit code 1 is a version-based warning, not proof that the installed package is unpatched. HTTP headers cannot tell whether the version contains a backported fix. Describe the result as potentially vulnerable, never confirmed vulnerable. An `ok` result with exit code 0 means only that the advertised stable version meets an upstream fixed release: 8.2.35, 8.3.36, 8.4.27 or 8.5.12. The installed build is not verified. Missing or unrecognized versions, prereleases and failed checks return `unknown` with exit code 3. Headers can be changed. A server-side check of the package serving the site is stronger evidence. Do not run exploit probes. Exit codes 1 and 3 are check results. Invalid arguments exit with code 2.