Function project_dir 

pub fn project_dir(root: Option<&str>) -> Option<PathBuf>

Resolve the project directory for session storage.

Mirrors zag’s Config::resolve_project_dir (zag-agent/src/config.rs:188):

1. If root is provided, sanitize it directly.
2. Otherwise locate the git repository root containing cwd.
3. Otherwise fall back to the global base directory (no project subdir).