docs: add project changelog

AGENTS.md

@@ -48,6 +48,7 @@ This repository is a browser-first photo mapping prototype.
 - Make small, focused changes.
 - Commit every git change set before finishing a task.
 - Use conventional commit messages.
+- Document each committed change set in `Changelog.md` with a short summary.
 - Verify the result with `git diff --check` when possible.
 - Update the README only when the user-facing setup or features change.