Replace syscall.Rmdir with golang.org/x/sys for cross-platform directory removal

[rumpl]

Summary

Replace syscall.Rmdir with platform-specific implementations using golang.org/x/sys, the supported successor to the frozen syscall package.

Changes

• rmdir_unix.go — uses unix.Rmdir on aix/darwin/dragonfly/freebsd/linux/netbsd/openbsd/solaris
• rmdir_windows.go — uses windows.RemoveDirectory
• filesystem.go — calls the new rmdir() helper, removes syscall and errors imports
• filesystem_test.go — relaxes assertion to match OS-level error string ("not a directory" vs "is not a directory")

Both unix.Rmdir and windows.RemoveDirectory are atomic, directory-only syscalls — they will not accidentally delete regular files and avoid the TOCTOU race that an os.Lstat + os.Remove sequence would introduce.

golang.org/x/sys is already a dependency of this project.

Fixes #1884

[docker-agent]

Review Summary

This PR successfully modernizes the directory removal implementation by replacing the frozen syscall package with platform-specific implementations from golang.org/x/sys. The changes are well-structured with proper build tags and maintain atomicity guarantees.

However, there is one notable behavioral change in error handling that should be considered.

[docker-agent]

Loss of type-safe ENOTDIR error handling

The old implementation explicitly checked for syscall.ENOTDIR using errors.Is() to distinguish "not a directory" errors from other failure types (permission denied, path not found, etc.). The new implementation removes this check and now relies on error message strings.

While the underlying system calls (unix.Rmdir and windows.RemoveDirectory) do return appropriate error types/messages, callers that previously depended on type-safe error detection would now need to parse error strings. This is more fragile and platform-dependent.

Impact: Moderate - the functionality still works and the test passes, but error handling becomes less robust and harder to reason about programmatically.

Consider: If maintaining the explicit ENOTDIR check is important for API consumers, you could wrap the platform-specific errors and check for the appropriate error types (unix.ENOTDIR on Unix, windows.ERROR_DIRECTORY on Windows) before returning the generic error message.