Add commit-msg hook refusing AI attribution

.githooks/commit-msg

@@ -0,0 +1,31 @@
+#!/bin/sh
+#
+# Refuses AI attribution in commit messages.
+#
+# This repo is public, so history is the first thing an outside contributor
+# sees. Attribution cannot be removed retroactively without rewriting every
+# commit after it, and clones have already copied it.
+#
+# Enable after cloning:
+#
+#     git config core.hooksPath .githooks
+#     chmod +x .githooks/commit-msg
+#
+# The Gitea web API cannot set the executable bit, so the chmod is required
+# on first clone even though the file is committed.
+
+msg_file="$1"
+
+# Co-author trailers naming an assistant.
+if grep -qiE 'co-?authored-?by:[[:space:]]*.*(claude|anthropic|copilot|cursor|codex|chatgpt|gpt-|gemini|aider|devin|llm|\bai\b)' "$msg_file"; then
+	echo "REFUSED: AI co-author trailer in commit message." >&2
+	exit 1
+fi
+
+# Generated-with footers and the robot emoji.
+if grep -qiE 'generated with|written by (claude|an? ai)|🤖' "$msg_file"; then
+	echo "REFUSED: AI attribution in commit message." >&2
+	exit 1
+fi
+
+exit 0