docs(changelog): rewrite all release notes into friendly New Features / Fixes format

Distill every release's engineer-facing entry into plain-language, user-readable
notes (New Features / Fixes, with Breaking Changes / Security surfaced where they
apply). Also reconcile the mislabeled [0.7.8] block to [0.7.9] to match the
published GitHub release tag and fix its dead link reference.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>