feat(python): add direct VFS convenience methods on Bash/BashTool

[chaliy]

Context

Node bindings expose VFS operations directly on the Bash class (bash.readFile(), bash.writeFile(), bash.ls(), bash.glob(), etc.) for ergonomic access. Python requires an extra indirection: bash.fs().read_file().

Both patterns should be available in Python: direct methods for convenience, fs() accessor for advanced use.

This is part of the Python ↔ Node binding parity effort (Phase 1 — API Parity).

What to implement

Add 13 delegate methods to PyBash and PyBashTool in crates/bashkit-python/src/lib.rs that forward to the internal FileSystem:

bash.read_file(path: str) -> str
bash.write_file(path: str, content: str) -> None
bash.append_file(path: str, content: str) -> None
bash.mkdir(path: str, recursive: bool = False) -> None
bash.exists(path: str) -> bool
bash.remove(path: str, recursive: bool = False) -> None
bash.stat(path: str) -> dict
bash.chmod(path: str, mode: int) -> None
bash.symlink(target: str, link: str) -> None
bash.read_link(path: str) -> str
bash.read_dir(path: str) -> list[dict]
bash.ls(path: str = ".") -> list[str]       # simple name list
bash.glob(pattern: str) -> list[str]         # safe glob matching

Important design notes

• These should return str (not bytes like FileSystem.read_file()), matching Node's behavior and the common case
• ls() returns a simple list[str] of names (like Node's ls()), not the full metadata that read_dir() returns
• glob() must use safe glob only (no arbitrary shell expansion)
• The fs() accessor remains available for users who need bytes-level access or the FileSystem static constructors
• Methods should be added to both PyBash AND PyBashTool

Node reference

In crates/bashkit-js/src/lib.rs and wrapper.ts, Node implements these as methods on the Bash class that delegate to the internal interpreter's VFS.

Acceptance criteria

• All 13 methods added to PyBash class
• All 13 methods added to PyBashTool class
• read_file() returns str (UTF-8 decoded), raises on binary/non-UTF-8
• write_file() and append_file() accept str
• ls() returns list[str] of entry names in the given directory
• glob() returns list[str] of matching paths (safe glob only)
• stat() returns dict with keys: file_type, size, mode, modified, created
• read_dir() returns list[dict] with name and metadata per entry
• All methods raise appropriate errors for invalid paths / missing files
• Methods work correctly after reset() (VFS is cleared)
• Methods reflect changes made by bash command execution (e.g., bash.execute_sync("echo hi > /tmp/f") then bash.read_file("/tmp/f"))
• Type stubs updated in _bashkit.pyi with docstrings
• Tests for each method (positive + negative cases)
• cargo clippy and ruff check pass
• README updated showing direct method usage