adds docs/m2pack-integration.md covering the signature boundary,
runtime key env-var delivery, telemetry opt-in, backward compatibility
and expected on-disk layout. README gains a short "Release formats"
section pointing at the new doc, and CHANGELOG tracks the [Unreleased]
entries.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
