Setup Caveman

One-time setup. Adds a single line to the project's CLAUDE.md enabling caveman mode by default.

What it does

Append this line to CLAUDE.md (create the file if it doesn't exist):

Use caveman mode by default (see ~/.claude/skills/caveman/SKILL.md). Stop with "stop caveman".

Key rules

• Never overwrite existing CLAUDE.md content -- only append
• If the line already exists, skip