Shared runtime packages used by the independently published command modules.This module is published as cli/core and is an implementation dependency forcli/<command> modules. The portable filesystem layer exposes file-kind andregular-file-size reads, atime/mtime/ctime timestamps, access checks, andsymbolic-link creation on Native/Wasm. It also centralizes nanosecond comparison,age buckets, update/backup decisions, and preflighted copy traversal. Permissionreads, arbitrary timestamp setters, hard links, readlink, special-file creation,and EXDEV classification remain intentionally unavailable.