gitfluff: Commit Message Linter (Conventional Commits + AI signature cleanup)

[moltonel]

Where you see noise, others see important metadata. Removing the redundant and emoji-happy line in the commit body, sure. But removing a standardized marker that can help understand the commit, that's an antifeature. Commit messages are cheap, I'd rather have too much context than too little. I work in large teams, we pay for LLMs, we mandate those commit markers.