skill_export.py imported the POSIX-only stdlib module `resource`
unconditionally at module level, so the benchmark entry point died with
ModuleNotFoundError on Windows before argument parsing. Mirror the
try/except ImportError guard already established by
checkpoint/checkpoint_bench_common.py, report "peak_rss_bytes": null when
the module is unavailable, and leave POSIX scaling unchanged (macOS
bytes, Linux KiB scaled to bytes).
Fixes#7380