feat(cli): expose the pack documentation sync command

[guyua9]

Summary

• expose the existing pack documentation sync helper as rulebook-ai sync-docs
• accept an explicit --project-dir so source and destination roots are deterministic
• validate all source files before copying and add unit/install coverage

Tests

• PYTHONPATH=src pytest -o addopts="" tests/unit/test_sync_pack_docs.py -q (3 passed)
• isolated editable install with PyYAML: pytest -o addopts="" tests/integration/test_package_installation.py -q (7 passed)
• python3 -m compileall
• git diff --check