pub fn data_dir() -> Result<PathBuf, String>Expand description
Data directory — sessions, vectors, graphs, knowledge, archives, memory.
Delegates to lean_ctx_data_dir, which since GL #606 defaults fresh
installs to $XDG_DATA_HOME/lean-ctx. Legacy ~/.lean-ctx and pre-split
mixed $XDG_CONFIG_HOME/lean-ctx installs (and an explicit
LEAN_CTX_DATA_DIR) continue to resolve in place for backward compatibility.