Default Co-Authored-By line in commits should be opt-in, not opt-out

Problem

When creating git commits, Claude Code adds a Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com> line by default. This is baked into the system prompt.

Why this is problematic

1. Tool attribution on outputs is unusual: Photoshop doesn't watermark photos. Compilers don't sign binaries. IDEs don't add comments crediting themselves. Paying for a tool doesn't mean outputs get branded.

1. Opt-out creates configuration rot: Users who don't want this must add overrides to their CLAUDE.md. This wastes configuration space on negating defaults rather than encoding user intent.

1. Correction vs guidance asymmetry: Good defaults flow with user intent. Requiring users to configure around unwanted behavior is thermodynamically wasteful—it's correction, not guidance.

1. The work product belongs to the user: The user constructs the context, provides requirements, reviews output, and takes responsibility. The tool assists but doesn't co-author in any meaningful sense.

Suggested fix

• Remove the default Co-Authored-By instruction from the system prompt
• Make it opt-in via user configuration for those who want it
• Or at minimum, make it configurable without requiring CLAUDE.md overrides (e.g., a setting)

Current workaround

Tell Claude not to add it, or add override text to CLAUDE.md—both suboptimal.