chore(hooks): reject tooling co-author trailers at commit time

The voice rule states that commits are authored by the maintainer's git
identity alone. Editors and CLIs append co-author trailers and session
links by default, and once such a line is in the object store only a
history rewrite removes it — so enforce it before the commit exists.

The guard rejects tooling co-author trailers, session links, and
generated-with lines; human co-authors and ordinary messages pass.

lefthook.yml

@@ -16,6 +16,24 @@
 # Per project policy (`--no-verify` is banned), prefer fixing the
 # finding over bypassing the hook.
 
+# Commit-message guard: the project voice rule (CLAUDE.md, "Voice and
+# privacy") states that commits are authored by the maintainer's git
+# identity alone and carry no tooling co-author trailer or session link.
+# Editors and CLIs add those trailers by default, and a message that
+# picks one up is only removable from the object store by a history
+# rewrite — so the check belongs here, before the commit exists, not in
+# a review pass afterwards.
+commit-msg:
+  commands:
+    no-tooling-trailer:
+      run: |
+        if grep -qiE '^(Co-Authored-By:.*(claude|anthropic|copilot|gpt|gemini)|Claude-Session:|Generated with)' "{1}"; then
+          echo "ERROR: the commit message carries a tooling co-author trailer or session link." >&2
+          echo "Project policy: commits are authored by the maintainer's git identity alone." >&2
+          echo "Remove the offending line(s) from the message and commit again." >&2
+          exit 1
+        fi
+
 pre-commit:
   commands:
     gitleaks: