📖✨：say how a pull request reaches the history

The handbook described the rules a message answers to without saying
what happens to it afterwards. Labelling a pull request `commit-queue`
squashes it into one commit whose body keeps every message it contained.

Signed-off-by: Derek Lewis <DerekNonGeneric@inf.is>
Assisted-by: Claude-Code:claude-opus-5

Author: DerekNonGeneric

collections/_docs/handbook/style/commit-messages.md

@@ -10,6 +10,11 @@ The rules below are checked by `nps verify.commits`, which runs on every pull
 request. Nothing here is a matter of taste that the checker is unaware of: if it
 passes, the message conforms.
 
+What lands is built from these messages rather than typed again at merge time.
+Labelling a pull request `commit-queue` squashes it into one commit whose body
+keeps every message it contained, with the trailers gathered where git reads
+them and `PR-URL` added.
+
 ## Subject
 
 The first line names a classification and then says what the change does: