Co-Authored-By attribution should be opt-in, not opt-out

Description

Claude Code adds a Co-Authored-By: Claude <noreply@anthropic.com> trailer to every git commit by default. This should be off by default and opt-in for users who want it.

Why

• Users don't expect their CLI tool to inject branding into their git history
• It acts as free marketing/advertising in every repo, commit log, and GitHub history
• Most developer tools don't inject attribution into user commits by default
• It clutters commit messages with information the developer didn't author
• The current behavior requires users to discover and configure the attribution setting to disable it

Expected behavior

Attribution should be opt-in: off by default, with a setting to enable it for users who want transparency about AI-assisted commits.