Kernel-style AI commit attribution (Assisted-by) — skill + enforcing PreToolUse hook.
Credit AI assistance with an `Assisted-by:` trailer per the Linux-kernel coding-assistants policy. Ships the `git-attribution` skill plus a PreToolUse hook that blocks `git commit` commands that author a message without the trailer, or that add an AI `Signed-off-by:` / `Co-Authored-By: Claude`.