chore(cursor): add rule banning Co-authored-by trailers on commits

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: elkaix

.cursor/rules/no-co-author-trailers.mdc

@@ -0,0 +1,31 @@
+---
+description: Never add Co-authored-by or other AI/tool trailers to git commits or PRs
+alwaysApply: true
+---
+
+# No co-author or AI trailers on commits
+
+**Never** add `Co-authored-by`, `Signed-off-by` for an AI tool, or any Cursor/Claude/Copilot
+footer to commit messages or PR descriptions.
+
+This matches `AGENTS.md`: commits use Conventional Commits subject (+ optional body) only.
+
+## When creating commits
+
+- Pass the message via HEREDoc or `-m` with **only** the intended subject/body.
+- Do **not** append `Co-authored-by: Cursor <cursoragent@cursor.com>` or similar.
+- If a hook or tool adds a co-author trailer, **amend it out** before pushing (only when amend rules allow).
+
+## Examples
+
+```text
+feat(soul): emit TodoListUpdated wire event
+```
+
+Not:
+
+```text
+feat(soul): emit TodoListUpdated wire event
+
+Co-authored-by: Cursor <cursoragent@cursor.com>
+```