Fix CLI 404: replace importlib.resources.files() with pathlib.__file__ for package directory resolution

[Copilot]

importlib.resources.files("docbuddy") behaves inconsistently across Python versions and install methods — str() on the returned traversable can resolve to unexpected paths, causing the HTTP server to serve the wrong directory and standalone.html to return 404.

Changes

• cli.py: Replace importlib.resources.files() with pathlib.Path(__file__).parent, which unconditionally resolves to the installed package directory. Extract into _pkg_dir() helper for testability. Drop the now-unused importlib.resources import.

# Before — unreliable on some Python versions / install methods
pkg_ref = files("docbuddy")
pkg_dir = str(pkg_ref)  # str() may not yield a valid filesystem path

# After — always points to the directory containing the installed cli.py
def _pkg_dir() -> pathlib.Path:
    return pathlib.Path(__file__).parent

• tests/test_plugin.py: Update CLI tests to monkeypatch _pkg_dir instead of files. Use tmp_path fixture for filesystem-based error-path testing. Add assertion that directory= kwarg passed to SimpleHTTPRequestHandler matches the expected package path.

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