Add a CHANGELOG #15

Seven tags and no release notes. The wire contract has moved twice (v3 to v4 to v5) with nothing recording when or why.

Done when:

• CHANGELOG.org covers every tagged release, written from the repository's history rather than invented.
• Every SCHEMA_VERSION move is called out, since that is the compatibility contract between the daemon, the Swift collector and every renderer.
• The README points at it.