docs(release): 0.4.7 release notes

[eli64s]

Summary

Release notes for v0.4.7, the stabilization cut before the diagram-skill release. What ships:

• Custom themes are validated consistently and can no longer inject markup or CSS.
• Matrix tables wrap content or reject oversized layouts instead of clipping them.
• POST /v1/compose accepts every public compose option.
• Python 3.10 is supported.

Scope

Only CHANGELOG.md changes; runtime behavior, generated artifacts, and package size are unchanged.

Validation

Check	Result
Content	Every previous change remains represented and was checked against the related code and tests
Markdown	git diff --check HEAD^..HEAD passes
Tests	Not run; documentation-only change