docs: streamline contributor profile documentation

[rrbe]

Summary

• replace the contributor signal planning notes with a concise description of the shipped Contributor Profile Card
• remove the resolved contributor-card debugging handoff and update code references to the new document
• make CLAUDE.md a symlink to AGENTS.md so agent guidance has one source of truth

Validation

• pnpm test (224 passed)
• pnpm build
• pnpm typecheck
• pnpm exec oxfmt --check docs/contributor-profile-card.md
• git diff --check