docs: rewrite CHANGELOG with user-facing historical entries

Replace auto-generated commit dumps and orphaned [Unreleased] section
with polished, outcome-focused entries for every version from v1.0.0
through v4.5.0. Features describe what changed for adopters, not
internal commit shorthand.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: davidmatousek