Add a commit-msg hook for this project's attribution form

card-games is published, so its commit messages are permanent public
record.  The new hook refuses a session-link trailer and a
Co-Authored-By trailer naming Claude or anthropic.com, and checks that
any Assisted-by trailer reads NAME (for PERSON; MODEL).  A message with
no Assisted-by line passes: attribution stays the author's call.

* hooks/commit-msg: New hook.
* hooks/applypatch-msg: New hook; runs commit-msg for git am.
* test/commit-msg-tests.el: New ERT tests that drive the sh hook.
* Makefile (GITHOOKS): New variable.
(hooks): Install all three hooks.
(test-hooks): New target.
(check): Run it.

Assisted-by: Mentat (for Corwin; claude-opus-5-5)

hooks/applypatch-msg

@@ -0,0 +1,11 @@
+#!/bin/sh
+# hooks/applypatch-msg -- give patches applied with `git am' the same
+# trailer check as an ordinary commit, by running hooks/commit-msg.
+#
+# Install:  make hooks
+
+hook=$(git rev-parse --git-path hooks/commit-msg)
+if [ -x "$hook" ]; then
+    exec "$hook" "$@"
+fi
+exit 0