Function get_active_entries_dir

pub fn get_active_entries_dir(
    notebook_override: Option<String>,
) -> Result<PathBuf>

Gets the entries directory for the currently active notebook.

This is the core of the multi-notebook feature. It resolves the path based on this priority:

1. The --notebook command-line flag (passed in as notebook_override).
2. The RJOT_ACTIVE_NOTEBOOK environment variable.
3. The “default” notebook if neither is set.

It will automatically create the notebook directory if it doesn’t exist.