Remove doc/release-notes/, CHANGELOG.md is now the source of truth

Every per-release file has been folded into CHANGELOG.md (commit 06d5328).
Delete the directory and update the make-release skill so future
releases prepend a new section to CHANGELOG.md instead of creating a
new RELEASE-NOTES-X.Y.Z.md file. Update CHANGELOG.md's intro line
which previously claimed the per-release files were authoritative.

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