core/memory+cli: remember session LLM; readline prompts

Store provider_name/model on sessions (SQLite migrate); restore on
resume and persist on create//model//provider. Startup prefers session
selection over re-asking. ask/choose and provider/model pick use
readline with Tab completion when available.

tests/test_cli/test_export.py

@@ -38,7 +38,11 @@ def test_session_export_json_roundtrip() -> None:
         created_at=now,
         updated_at=now,
         messages=messages,
+        provider_name="local",
+        model="tiny",
     )
+    assert payload.get("provider") == "local"
+    assert payload.get("model") == "tiny"
     assert isinstance(payload, dict)
     raw = encode_session_export_json(payload)
     assert "7" in raw

tests/test_cli/test_repl_commands.py

@@ -173,6 +173,23 @@ async def test_rename_slash(state: ReplState) -> None:
     assert row.name == "my-chat"
 
 
+async def test_model_switch_persists(state: ReplState) -> None:
+    from plyngent.config.models import ModelConfig
+
+    state.provider.models = {
+        "m1": ModelConfig(),
+        "m2": ModelConfig(),
+    }
+    state.model = "m1"
+    await state.persist_llm_selection()
+    assert await handle_slash(state, "/model m2") is True
+    assert state.model == "m2"
+    assert state.session_id is not None
+    row = await state.memory.get_session(state.session_id)
+    assert row is not None
+    assert row.model == "m2"
+
+
 async def test_delete_slash_confirm(
     state: ReplState,
     capsys: pytest.CaptureFixture[str],

tests/test_cli/test_selection.py

@@ -38,11 +38,23 @@ def test_select_model_from_list() -> None:
     assert select_model(provider, preferred="m1") == "m1"
 
 
-def test_select_model_prompt_when_empty(monkeypatch: pytest.MonkeyPatch) -> None:
+def test_select_model_prompt_when_empty() -> None:
+    from plyngent.prompting import temporary_backend
+    from tests.test_prompting import ScriptedBackend
+
     provider = OpenAIProvider(access_key_or_token="sk")
+    with temporary_backend(ScriptedBackend(["gpt-test"])):
+        assert select_model(provider) == "gpt-test"
 
-    def _prompt(*_args: object, **_kwargs: object) -> str:
-        return "gpt-test"
 
-    monkeypatch.setattr("click.prompt", _prompt)
-    assert select_model(provider) == "gpt-test"
+def test_select_provider_interactive_choose() -> None:
+    from plyngent.prompting import temporary_backend
+    from tests.test_prompting import ScriptedBackend
+
+    providers = {
+        "a": OpenAIProvider(access_key_or_token="sk"),
+        "b": OpenAICompatibleProvider(access_key_or_token="sk", url="https://x/v1"),
+    }
+    with temporary_backend(ScriptedBackend(["2"])):
+        name, _ = select_provider(providers)
+    assert name == "b"