Add a CHANGELOG

[thorwhalen]

Add a CHANGELOG

The package has no CHANGELOG.md. The wads CI already bumps the version on every merge
(0.0.14 → 0.0.22 over the post-v1 pass), so a changelog would let users and downstreams
(braidio, nw) see what changed per release.

Scope

• Add CHANGELOG.md (Keep-a-Changelog format), seeded with the v1 → current history
  (foundation, index, ingestion, select, weave, MCP, eval, the post-v1 hardening + AI-first).
• Optionally wire the wads changelog tooling (wads-changelog) so entries are generated /
  appended on release, and link it from the README + docs.

Acceptance

• CHANGELOG.md exists, covers the shipped versions, and is referenced from the README.