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.

src/plyngent/cli/app.py

@@ -82,12 +82,21 @@ def _warn_recoverable_providers(recoverable: Mapping[str, object]) -> None:
 
 def _database_config(store: ConfigStore, *, quiet: bool = False) -> DatabaseConfig:
     raw = dict(store.database)
-    # Prefer a durable file DB so sessions survive CLI restarts.
-    if raw.get("url") in {None, "", ":memory:"} and raw.get("implementation", "sqlite") == "sqlite":
+    url = raw.get("url")
+    impl = raw.get("implementation", "sqlite")
+    # Unset/empty → durable user-data chat.db so interactive sessions persist.
+    # Explicit ``:memory:`` is kept as-is (ephemeral; warn so it is not accidental).
+    if impl == "sqlite" and url in {None, ""}:
         db_path = user_data_path("plyngent", ensure_exists=True) / _DEFAULT_DB_FILENAME
         raw = {**raw, "implementation": "sqlite", "url": str(db_path)}
         if not quiet:
             click.secho(f"using database: {db_path}", fg="bright_black", err=True)
+    elif not quiet and impl == "sqlite" and url == ":memory:":
+        click.secho(
+            "warning: database url is :memory: — sessions are not persisted to disk",
+            fg="yellow",
+            err=True,
+        )
     return msgspec.convert(raw, DatabaseConfig)
 
 

src/plyngent/cli/editor.py

@@ -18,10 +18,12 @@ _MINIMAL_CONFIG = """\
 # plyngent configuration
 # edit providers below
 
-# Optional: omit [database] to use ~/.local/share/plyngent/chat.db (Linux).
+# Optional: omit [database] (or leave url unset) → ~/.local/share/plyngent/chat.db.
+# url = ":memory:" keeps a true in-memory SQLite (CLI warns; nothing on disk).
 # [database]
 # implementation = "sqlite"
 # url = "/path/to/chat.db"
+# # url = ":memory:"
 
 # [agent]
 # system_prompt = "You are a careful coding assistant."

src/plyngent/config/models.py

@@ -4,10 +4,15 @@ from msgspec import Struct, field
 
 
 class DatabaseConfig(Struct, omit_defaults=True):
-    """Database connection configuration."""
+    """Database connection configuration.
+
+    ``url`` is ``None`` when unset. The CLI fills a durable user-data
+    ``chat.db`` only for unset/empty url. Explicit ``":memory:"`` is a true
+    in-memory SQLite (no rewrite).
+    """
 
     implementation: str = "sqlite"
-    url: str = ":memory:"
+    url: str | None = None
     username: str | None = None
     password: str | None = None
 

src/plyngent/memory/database/engine.py

@@ -20,7 +20,7 @@ def build_async_url(config: DatabaseConfig) -> str:
         raise UnsupportedDatabaseError(msg)
 
     url = config.url
-    if url in {":memory:", ""}:
+    if url is None or url in {":memory:", ""}:
         return "sqlite+aiosqlite:///:memory:"
     if url.startswith("sqlite+aiosqlite://"):
         return url