Having re-looked, I just found 'git rev-parse --verify MERGE_HEAD' which may help here. Time to do some testing. It would be good to clean up and standardise the commit msg itself too. Thanks.
The prepare-commit-msg hook is the one. One of the hook args is the type of commit (merge, squash, etc). And the merge commit msg files exist (SQUASH_MSG, MERGE_MSG) so my original logic all works.
$ git log -1
commit 735ce5f9998736c4066d63ab851df2023640dad5 (HEAD -> master)
Author: Doug Bridgens <thisdougb@users.noreply.github.com>
Date: Tue Apr 1 22:31:22 2025 +0100
test4 does some stuff
Time spent on feat_test4: 0d:0h:7m secs=426