docs: use release: for squash-merged release pull requests

A release spans a fix, a feature and some chores at once, so no single
conventional type describes it honestly. release: makes main's log one line
per deployment, which is what that branch is for, with the pull request body
as the notes.

CONTRIBUTING.md

@@ -59,8 +59,21 @@ alternative was — the diff already says what changed. If a fix was subtle,
 say what made it subtle; if a test caught something, say what. Commits here
 are the only design record this project has.
 
+### Release pull requests
+
 Because `main` is squash-merged, a pull request title becomes a commit message
-on `main`. Give it the same treatment.
+on `main`. A release spans a fix, a feature and some chores at once, so none
+of the types above fits it honestly. Use `release:` instead:
+
+```
+release: repair the catalog 404 and stop the page jumping
+```
+
+`main`'s log is then one line per deployment, which is what that branch is
+for, and the pull request body serves as the release notes. No version in the
+title — the CalVer tag is not created until `make image` runs after the merge.
+
+The types above are for `dev`, where a commit really does do one thing.
 
 ## Things that are easy to get wrong