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)

test/commit-msg-tests.el

@@ -0,0 +1,76 @@
+;;; commit-msg-tests.el --- ERT tests for hooks/commit-msg  -*- lexical-binding: t; -*-
+
+;; Run with:  make test-hooks   (or)
+;;   emacs -Q --batch -l test/commit-msg-tests.el -f ert-run-tests-batch-and-exit
+;;
+;; The hook is POSIX sh; these tests drive it from Emacs Lisp, so the
+;; check and its tests are written in different languages.  Each test
+;; feeds one message file to the hook and reads only its exit status.
+;; The table holds messages that must pass and messages that must be
+;; refused, so a hook that accepts everything, or refuses everything,
+;; fails here.
+
+;;; Code:
+
+(require 'ert)
+
+(defconst cmt--hook
+  (expand-file-name "../hooks/commit-msg"
+                    (file-name-directory (or load-file-name buffer-file-name)))
+  "The hook under test.")
+
+(defun cmt--status (message)
+  "Return the exit status of the hook run on MESSAGE."
+  (let ((file (make-temp-file "commit-msg-test-")))
+    (unwind-protect
+        (progn
+          (let ((coding-system-for-write 'no-conversion))
+            (with-temp-file file (insert message)))
+          (call-process "sh" nil nil nil cmt--hook file))
+      (delete-file file))))
+
+(defconst cmt--subject "Fix a thing\n\nBody text.\n\n")
+
+(defconst cmt--accept
+  `(("no trailer at all"        . "Fix a thing\n\nBody text.\n")
+    ("persona form, semicolon"  . ,(concat cmt--subject "Assisted-by: Mentat (for Corwin; claude-opus-5-5)\n"))
+    ("persona form, comma"      . ,(concat cmt--subject "Assisted-by: Mentat (for Corwin, claude-opus-5-5)\n"))
+    ("key case varies"          . ,(concat cmt--subject "Assisted-By: Mentat (for Corwin; claude-opus-5-5)\n"))
+    ("CRLF line endings"        . "Fix a thing\r\n\r\nAssisted-by: Mentat (for Corwin; claude-opus-5-5)\r\n")
+    ("no final newline"         . ,(concat cmt--subject "Assisted-by: Mentat (for Corwin; claude-opus-5-5)"))
+    ("comment line ignored"     . ,(concat cmt--subject "# Claude-Session: https://example.invalid/x\n"))
+    ("after the scissors line"  . ,(concat cmt--subject
+                                           "# ------------------------ >8 ------------------------\n"
+                                           "+Claude-Session: https://example.invalid/x\n"
+                                           "+Co-Authored-By: Claude <noreply@anthropic.com>\n")))
+  "Messages the hook must accept, by name.")
+
+(defconst cmt--refuse
+  `(("session link"             . ,(concat cmt--subject "Claude-Session: https://claude.ai/code/session_x\n"))
+    ("session link, lower case" . ,(concat cmt--subject "claude-session: https://claude.ai/code/session_x\n"))
+    ("co-author, Claude"        . ,(concat cmt--subject "Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>\n"))
+    ("co-author, lower case"    . ,(concat cmt--subject "co-authored-by: Someone <bot@anthropic.com>\n"))
+    ("older Assisted-by form"   . ,(concat cmt--subject "Assisted-by: Claude:claude-opus-4-8\n"))
+    ("persona form, no model"   . ,(concat cmt--subject "Assisted-by: Mentat (for Corwin)\n"))
+    ("persona form, no person"  . ,(concat cmt--subject "Assisted-by: Mentat (claude-opus-5-5)\n"))
+    ("good line then a bad one" . ,(concat cmt--subject
+                                           "Assisted-by: Mentat (for Corwin; claude-opus-5-5)\n"
+                                           "Claude-Session: https://claude.ai/code/session_x\n")))
+  "Messages the hook must refuse, by name.")
+
+(ert-deftest cmt-accepts ()
+  "Every message in `cmt--accept' passes the hook."
+  (skip-unless (executable-find "sh"))
+  (dolist (case cmt--accept)
+    (should (equal (cons (car case) 0)
+                   (cons (car case) (cmt--status (cdr case)))))))
+
+(ert-deftest cmt-refuses ()
+  "Every message in `cmt--refuse' is refused by the hook."
+  (skip-unless (executable-find "sh"))
+  (dolist (case cmt--refuse)
+    (should (equal (cons (car case) 1)
+                   (cons (car case) (cmt--status (cdr case)))))))
+
+(provide 'commit-msg-tests)
+;;; commit-msg-tests.el ends here