Add a Phase 4 'm2pack commands' section to docs/cli.md with each
subcommand's flags and a pointer at the m2pack-secure repo for
installation. Update README.md with a short m2pack paragraph and
append the Phase 4 entry to the Unreleased CHANGELOG section.
Seed CHANGELOG.md in Keep a Changelog format, document every Phase 1
subcommand plus the --json flag placement fix as the initial 0.1.0
entry. Future minors and patches add sections above this one.