Add onboarding wizard framework and startup hook

src/config.py

@@ -111,6 +111,13 @@ def load(path: str | None) -> Config:
     return cfg
 
 
+def save(path: str | Path | None, cfg: Config) -> Path:
+    validate(cfg)
+    target = Path(path) if path else DEFAULT_CONFIG_PATH
+    _write_default_config(target, cfg)
+    return target
+
+
 def redacted_dict(cfg: Config) -> dict[str, Any]:
     return asdict(cfg)