Fix php_uname() ValueError on PHP 8.4+

[JoshSalway]

Fixes #106

Summary

OpenCommand passes PHP_OS (a string constant like "Darwin" or "Linux") as the mode parameter to php_uname():

$os = strtolower(php_uname(PHP_OS));

In PHP 8.4+, php_uname() strictly requires a single character mode ('a', 's', 'n', etc.), causing a ValueError. On PHP < 8.4, the multi-character string was silently accepted but produced undefined behavior (it only used the first character).

Since PHP_OS already contains the OS name (same value as php_uname('s')), the fix is to use it directly:

$os = strtolower(PHP_OS);

PHP_OS is a compile-time constant available since PHP 4, so this works on all supported versions (^8.1.0 per composer.json).

Tests

Added tests/Feature/OpenCommandTest.php with 3 tests:

• it can open a site in the browser — integration test exercising the full command with mocks
• it detects os without throwing value error — unit test confirming strtolower(PHP_OS) works without ValueError (the exact regression this PR fixes)
• it correctly identifies the current platform — validates OS detection returns darwin/linux/other

All 3 tests pass.

Test plan

• forge open works on macOS (PHP_OS = "Darwin")
• forge open works on Linux (PHP_OS = "Linux")
• Backwards compatible with PHP 8.1–8.3 (same runtime value as before)
• Fixes ValueError on PHP 8.4+
• Regression tests confirm fix

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