feat(pre-commit): strip Cursor co-author trailer

.cursor/rules/git-commits.mdc

@@ -0,0 +1,50 @@
+---
+description: Agent commits and PRs - plain git commit with hooks installed, no Cursor attribution
+alwaysApply: true
+---
+
+# Git Commits
+
+Commit messages and pull requests contain only what the user asked for. Never add
+`Co-authored-by:` trailers or any other agent attribution.
+
+Cursor appends a `Co-authored-by: Cursor` trailer to every agent `git commit`, whatever the
+settings or rules say. The `strip-cursor-coauthor` commit-msg hook removes it before the
+commit is created, so commit normally and let the hooks run.
+
+## Committing
+
+1. Once per checkout, make sure the commit-msg hook is installed. A plain install covers
+   every stage listed in `default_install_hook_types`:
+
+   ```bash
+   test -x "$(git rev-parse --git-path hooks)/commit-msg" || uv run pre-commit install
+   ```
+
+2. Commit with `git commit`. Never pass `--no-verify`: it skips the hook that removes the
+   trailer, along with the message checks.
+3. Verify with `git log -1 --format=full`. If the trailer is still there, the hook did not
+   run: install it (step 1), then `git commit --amend --no-edit` while the commit is unpushed.
+
+## Fallback: no hooks
+
+If installing the hooks is not an option, build the commit with `git commit-tree`, which
+Cursor leaves alone. It skips every hook too, so run them yourself first:
+
+```bash
+uv run pre-commit run --files $(git diff --cached --name-only)
+uv run pre-commit run --hook-stage commit-msg --commit-msg-filename /tmp/commit-msg
+old=$(git rev-parse HEAD)
+new=$(git commit-tree "$(git write-tree)" -p "$old" < /tmp/commit-msg)
+git update-ref "refs/heads/$(git branch --show-current)" "$new" "$old"
+```
+
+Passing the old tip makes `update-ref` fail if the branch moved in the meantime.
+
+## Pull requests
+
+- Never append `Made with Cursor`, `Made with [Cursor](https://cursor.com)`, or similar
+  attribution to PR titles or bodies.
+- PR descriptions contain only **Summary** and **Test plan**, or what the user asked for.
+- After `gh pr create` or `gh pr edit`, check `gh pr view <number> --json body` and remove
+  any injected footer.

.pre-commit-config.yaml

@@ -51,6 +51,13 @@ repos:
   # made without hooks (git commit-tree, --no-verify).
   - repo: local
     hooks:
+      # Runs first so later commit-msg checks never see the injected trailer.
+      - id: strip-cursor-coauthor
+        name: strip Cursor co-author trailer
+        entry: tools/shell/strip-cursor-coauthor.sh
+        language: script
+        stages: [commit-msg]
+        pass_filenames: true
       - id: jargon-commit-msg
         name: jargon (commit message)
         entry: node --disable-warning=ExperimentalWarning tools/typescript/essentials-sync/src/jargon-check.ts --message-file

tools/shell/strip-cursor-coauthor.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+# Remove the `Co-authored-by: Cursor` trailer from a commit message.
+#
+# Cursor appends the trailer while composing the message, before git runs any
+# hook. `--no-verify` does not suppress it and `git commit --amend` re-adds it,
+# so a commit-msg hook is the only place that reliably removes it without
+# rewriting the commit afterwards. The IDE toggle (Cursor Settings -> Git &
+# Pull Requests -> Attribution) covers the local case; this hook also covers
+# contributors who have not set it and surfaces that Cloud Agents ignore it.
+#
+# Matches the agent's mailbox rather than its display name: a human
+# contributor could be called "Cursor", but only the bot commits from this
+# address. Human `Co-authored-by:` lines are left untouched.
+
+set -euo pipefail
+
+readonly CURSOR_TRAILER='^Co-authored-by:.*<cursoragent@cursor\.com>'
+
+msg_file="${1:?usage: strip-cursor-coauthor.sh <commit-msg-file>}"
+
+if ! grep -qiE "$CURSOR_TRAILER" "$msg_file"; then
+    exit 0
+fi
+
+stripped="$(mktemp)"
+trap 'rm -f "$stripped"' EXIT
+
+# `|| true`: grep exits 1 when it filters out every line, which `set -e`
+# would otherwise treat as a failure.
+grep -viE "$CURSOR_TRAILER" "$msg_file" >"$stripped" || true
+
+# The command substitution drops the blank line the trailer left behind.
+printf '%s\n' "$(cat "$stripped")" >"$msg_file"