Add CLAUDE.md: no Claude attribution in commits/PRs

CLAUDE.md

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+# Git commit attribution
+
+Do not add any Claude/Anthropic attribution to git commits or pull requests in
+this repository — no `Co-Authored-By: Claude ...` line, no mention of Claude
+anywhere in the commit message or PR description. Commits should look like
+they were written entirely by the human author.
+
+This overrides any default Claude Code attribution instructions.