Config file loader: `~/.config/fafycat/config.toml`

[davidchris]

Parent

#26

What to build

A new deep module that loads ~/.config/fafycat/config.toml and feeds path overrides into AppConfig. Affects every command (serve, import, init, and the future query commands). Independent of the CLI surface work — can ship in parallel.

• Public interface: load_config_file(path: Path | None) -> dict[str, str].
• Format: TOML via stdlib tomllib. No new dependency.
• Schema: a [paths] table with optional keys data_dir, db_url, model_dir, export_dir. All keys are optional; missing keys fall through to the next precedence layer.
• Resolution precedence (highest wins): --data-dir CLI flag > env var (FAFYCAT_DATA_DIR, FAFYCAT_DB_URL, etc.) > config file > built-in default (platformdirs).
• FAFYCAT_CONFIG env var overrides the config-file location.
• Missing config file is silent — fall through.
• Unknown keys produce a warning to stderr; do not crash.
• Malformed TOML or invalid path syntax fails loudly with a clear error.
• No auto-creation of the file on first run.

AppConfig is modified so its _default_* factories consult the loader between env vars and the platformdirs default.

Acceptance criteria

• load_config_file() returns an empty dict when the config file does not exist.
• load_config_file() returns the declared [paths] keys as a flat dict when the file is present and valid.
• Unknown keys in [paths] produce a stderr warning naming the key; the loader still returns the recognised keys.
• Malformed TOML raises a clear exception that is caught at the CLI boundary and surfaced as {"error": "..."} exit 1.
• Setting FAFYCAT_CONFIG=/some/path/config.toml causes the loader to read that path instead of the default.
• AppConfig resolves paths in the documented order: CLI flag > env > config file > default.
• An integration test sets each precedence layer and confirms the expected database URL ends up on AppConfig.database.url.
• Existing fafycat serve, fafycat import, fafycat init continue to work unchanged when the config file is absent.
• Tests are written before the implementation (red-green TDD).
• Pure-function unit tests cover: missing file, valid file, partial schema, unknown keys (warning surfaced via capsys/caplog), malformed TOML.
• No auto-creation: an integration test confirms the config file is not created when absent.

Blocked by

None - can start immediately.