feat(python): add direct VFS convenience methods

[chaliy]

Summary

Add direct text-oriented VFS convenience methods to the Python Bash and BashTool APIs so callers no longer need to go through fs() for common operations.

Closes #1257.

What Changed

• add read_file, write_file, append_file, mkdir, exists, remove, stat, chmod, symlink, read_link, read_dir, ls, and glob to both Python bindings
• reuse shared live-FS helpers so Bash and BashTool stay behaviorally aligned
• expose the new methods in _bashkit.pyi
• add regression coverage for happy paths, shell-state interaction, reset behavior, UTF-8 errors, and glob safety
• document the new convenience API in the Python README and specs/013-python-package.md

Verification

• python -m pytest crates/bashkit-python/tests/test_bashkit.py -q
• python -m pytest crates/bashkit-python/tests/test_bashkit.py -k 'direct_vfs_methods' -q
• uvx ruff check crates/bashkit-python
• uvx ruff format --check crates/bashkit-python
• smoke test via installed package: direct BashTool mkdir/write_file/append_file/read_file/ls/glob
• cargo clippy --all-targets --all-features -- -D warnings

Local Environment Notes

• cargo test --all-features currently fails on this machine in unrelated core tests: fs::realfs::tests::resolve_fallback_returns_normalized_path, fs::realfs::tests::resolve_fallback_validates_containment, and logging_impl::tests::test_script_formatting
• just pre-pr hit a local linker disk-space failure (errno=28) while building a test binary on a nearly full macOS volume