The `.brain` directory path is defined and resolved through two key locations:

Definition: In `config.py`, line 1-2, there's a constant that defines the base
name for the brain directory [config.py:1]:

    DB_PATH = '.brain/'
    BRAIN_DIR = Path(DB_PATH)

Resolution: In `main.py`, lines 120-145, within the `init_workspace` function,
this path is resolved by joining it with the workspace's absolute path [main.py:130]:

    path = Path(workspace_path).resolve()
    db_path = path / DB_PATH
    db_path.mkdir(parents=True, exist_ok=True)

The resolution process:
1. Takes the provided `workspace_path` parameter
2. Resolves it to an absolute path using `.resolve()`
3. Joins it with the `.brain/` directory constant
4. Creates the full directory structure if it doesn't exist