docs: seed CHANGELOG.md (Keep-a-Changelog format) (#51)

Closes Row-2 Phase 3 of the 2026-05-26 estate tech-debt audit chain
for this repo. The 2026-05-26 documentation-debt audit
(hyperpolymath/standards#197) flagged that 180 of 279 estate repos
lacked a CHANGELOG.md (65% gap) — this seed closes that finding here.

The seed:
  - Uses Keep-a-Changelog format with an [Unreleased] section.
  - Buckets the most recent 100 commits by conventional-commit prefix
    (feat/fix/refactor/docs/ci/build) into Added/Fixed/Changed/
    Documentation/CI sections.
  - References standards#206's changelog-reusable.yml + the canonical
    templates/cliff.toml for full-regeneration via git-cliff.

The file is initial — the maintainer can adopt changelog-reusable.yml
in this repo's CI to keep it auto-regenerated, or regenerate manually.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath