Add auto-updating README stats (project count + Lean LOC)

[Vilin97]

What

Surfaces two headline numbers in the main README.md and keeps them current automatically:

• project count — length of the projects list in LeanPool/projects.yml
• lines of Lean — wc -l-equivalent total across LeanPool/**/*.lean (build/dependency dirs excluded)

They render in a marked block right under the intro:

<!-- BEGIN STATS -->
**90** formalization projects · **676,138** lines of Lean
<!-- END STATS -->

How

• python/lean_pool/stats.py — computes the stats and rewrites the marked block (mirrors the existing render.py marker idiom and quality.py CLI style). Supports --check (fail-if-stale, no write) and --repo.
• .github/workflows/readme-stats.yml — on pushes to main that touch LeanPool/**, python/lean_pool/stats.py, or the workflow itself, regenerates the block and commits it back. The refresh commit touches only README.md, which is not in the paths: filter, so it can't re-trigger itself (no commit loop). Also runnable manually via workflow_dispatch.
• python/tests/test_stats.py — unit tests for counting, block replacement, idempotence, and --check.

Notes

• Stats refresh only when Lean source/projects change — an empty commit won't trigger it, by design.
• Pure tooling/infra change; touches no LeanPool/ content and no gates.

🤖 Generated with Claude Code

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.