Merge #986: Add v13.1.0 CHANGELOG entry

da31d40 docs: add v13.1.0 CHANGELOG entry (nymius)

Pull request description:

  Port CHANGELOG from #985 to master.


ACKs for top commit:
  apoelstra:
    ACK da31d40; successfully ran local tests


Tree-SHA512: 7ab99d9faa11c6ca0123abb661ae1789e1406abd291f25819d983409b76b762dc11db41be077b8bae5ca69eee9aeb58ccb02b3e9b4d4bc53da3d12fa2c8ed572

Author: apoelstra

CHANGELOG.md

@@ -1,3 +1,8 @@
+# 13.1.0 - June 9, 2026
+
+- plan: make `Plan`'s `descriptor` field public.
+  [#978](https://github.com/rust-bitcoin/rust-miniscript/pull/978)
+
 # 13.0.0 - October 22, 2025
 
 This release is a pretty big one and includes several significant refactors: