fix: package templates and simplify setup cache modes

[RexWzh]

Summary

• include mathlib template files in package builds so pip install leanup can run leanup setup without missing template errors
• simplify setup dependency modes to symlink or copy, and let cache preparation happen automatically when shared packages are missing
• refresh setup docs and tests to match the final cache reuse behavior

Testing

• pytest tests -q
• python -m build
• install the built wheel in a temporary venv and verify leanup/templates/mathlib/*.tmpl is present