chore: add lean4-benchmarks skill for Claude Code

This PR moves benchmark documentation from CLAUDE.md into a dedicated
project-level skill at .claude/skills/lean4-benchmarks/SKILL.md, covering
the measurement pipeline, variable-size benchmarks, JSONL format, custom
metrics, and combine.py behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
2 files changed