core/cli: keep database url :memory:; default url None

Unset/empty url still maps to durable user-data chat.db.
Explicit :memory: is not rewritten; CLI warns when not quiet.
DatabaseConfig.url default is None; engine treats None as memory.

tests/test_cli/test_app.py

@@ -1,10 +1,102 @@
-from __future__ import annotations
-
 from pathlib import Path
 
+import pytest
 from click.testing import CliRunner
 
-from plyngent.cli.app import main
+from plyngent.cli.app import _database_config, main
+from plyngent.config import load as load_config
+
+
+def test_database_config_memory_url_kept_with_warning(
+    tmp_path: Path,
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    """Explicit ``url = ":memory:"`` is not rewritten; CLI warns (when not quiet)."""
+    path = tmp_path / "mem.toml"
+    _ = path.write_text(
+        """
+[database]
+implementation = "sqlite"
+url = ":memory:"
+""",
+        encoding="utf-8",
+    )
+    store = load_config(path)
+    cfg = _database_config(store, quiet=False)
+    assert cfg.url == ":memory:"
+    assert cfg.implementation == "sqlite"
+    err = capsys.readouterr().err
+    assert ":memory:" in err
+    assert "not persisted" in err.lower() or "warning" in err.lower()
+
+
+def test_database_config_memory_url_quiet_no_warn(tmp_path: Path, capsys: pytest.CaptureFixture[str]) -> None:
+    path = tmp_path / "mem.toml"
+    _ = path.write_text(
+        """
+[database]
+implementation = "sqlite"
+url = ":memory:"
+""",
+        encoding="utf-8",
+    )
+    store = load_config(path)
+    cfg = _database_config(store, quiet=True)
+    assert cfg.url == ":memory:"
+    assert capsys.readouterr().err == ""
+
+
+def test_database_config_unset_url_uses_user_data(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Unset/empty url → durable user-data chat.db."""
+    data_dir = tmp_path / "user-data"
+    _ = data_dir.mkdir()
+
+    def fake_user_data_path(*_args: object, **_kwargs: object) -> Path:
+        return data_dir
+
+    monkeypatch.setattr("plyngent.cli.app.user_data_path", fake_user_data_path)
+    path = tmp_path / "empty-url.toml"
+    _ = path.write_text(
+        """
+[database]
+implementation = "sqlite"
+""",
+        encoding="utf-8",
+    )
+    store = load_config(path)
+    cfg = _database_config(store, quiet=True)
+    assert cfg.url == str(data_dir / "chat.db")
+
+
+def test_database_config_omitted_section_uses_user_data(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Omitted [database] → durable user-data chat.db (url default None)."""
+    data_dir = tmp_path / "user-data2"
+    _ = data_dir.mkdir()
+
+    def fake_user_data_path(*_args: object, **_kwargs: object) -> Path:
+        return data_dir
+
+    monkeypatch.setattr("plyngent.cli.app.user_data_path", fake_user_data_path)
+    path = tmp_path / "no-db.toml"
+    _ = path.write_text(
+        """
+[providers.local]
+preset = "openai-compatible"
+url = "http://127.0.0.1:1/v1"
+access_key_or_token = "x"
+""",
+        encoding="utf-8",
+    )
+    store = load_config(path)
+    assert store.database.get("url") is None
+    cfg = _database_config(store, quiet=True)
+    assert cfg.url == str(data_dir / "chat.db")
 
 
 def test_providers_list() -> None: