fix(pipeline): ground local-agent system prompt with current date (#2399)

The local-agent runner's system prompt is a static string with no
template-variable support, so the model had no anchor for "today" and
resolved relative time references (e.g. "this quarter", "latest")
against whichever period was best represented in training data instead
of the real date, sometimes confidently answering with stale
information for time-sensitive questions.

PreProcessor now appends a short, deterministically-computed
"Current date: ..." note to the system prompt on every request for
local-agent pipelines, alongside guidance to verify time-sensitive
facts with a search tool rather than answering from memory. The
existing skill-awareness prompt injection is refactored to share the
same append-to-system-prompt helper.

tests/unit_tests/pipeline/test_preproc.py

@@ -488,3 +488,115 @@ class TestPreProcessorToolSelection:
         result = await stage.process(query, 'PreProcessor')
 
         assert [tool.name for tool in result.new_query.use_funcs] == ['plugin_tool']
+
+
+class TestPreProcessorDateGrounding:
+    """Tests for current-date injection into the local-agent system prompt."""
+
+    @pytest.mark.asyncio
+    async def test_local_agent_appends_date_to_existing_system_message(self):
+        """Date grounding text should be appended to an existing system prompt."""
+        preproc = get_preproc_module()
+
+        app = FakeApp()
+        mock_session = make_session()
+        app.sess_mgr.get_session = AsyncMock(return_value=mock_session)
+
+        mock_conversation = Mock()
+        mock_conversation.prompt = Mock(messages=[])
+        mock_conversation.prompt.copy = Mock(return_value=Mock(messages=[]))
+        mock_conversation.messages = []
+        mock_conversation.uuid = None
+        app.sess_mgr.get_conversation = AsyncMock(return_value=mock_conversation)
+
+        app.model_mgr.get_model_by_uuid = AsyncMock(return_value=None)
+        app.tool_mgr.get_all_tools = AsyncMock(return_value=[])
+
+        from langbot_plugin.api.entities.builtin.provider import message as provider_message
+
+        system_message = provider_message.Message(role='system', content='You are a helpful assistant.')
+        mock_event_ctx = Mock()
+        mock_event_ctx.event = Mock(default_prompt=[system_message], prompt=[])
+        app.plugin_connector.emit_event = AsyncMock(return_value=mock_event_ctx)
+
+        stage = preproc.PreProcessor(app)
+        query = text_query('hello')
+
+        result = await stage.process(query, 'PreProcessor')
+
+        messages = result.new_query.prompt.messages
+        assert len(messages) == 1
+        assert messages[0].role == 'system'
+        assert messages[0].content.startswith('You are a helpful assistant.')
+        assert 'Current date:' in messages[0].content
+
+    @pytest.mark.asyncio
+    async def test_local_agent_creates_system_message_when_none_exists(self):
+        """A system message should be created when the prompt has none."""
+        preproc = get_preproc_module()
+
+        app = FakeApp()
+        mock_session = make_session()
+        app.sess_mgr.get_session = AsyncMock(return_value=mock_session)
+
+        mock_conversation = Mock()
+        mock_conversation.prompt = Mock(messages=[])
+        mock_conversation.prompt.copy = Mock(return_value=Mock(messages=[]))
+        mock_conversation.messages = []
+        mock_conversation.uuid = None
+        app.sess_mgr.get_conversation = AsyncMock(return_value=mock_conversation)
+
+        app.model_mgr.get_model_by_uuid = AsyncMock(return_value=None)
+        app.tool_mgr.get_all_tools = AsyncMock(return_value=[])
+
+        mock_event_ctx = Mock()
+        mock_event_ctx.event = Mock(default_prompt=[], prompt=[])
+        app.plugin_connector.emit_event = AsyncMock(return_value=mock_event_ctx)
+
+        stage = preproc.PreProcessor(app)
+        query = text_query('hello')
+
+        result = await stage.process(query, 'PreProcessor')
+
+        messages = result.new_query.prompt.messages
+        assert len(messages) == 1
+        assert messages[0].role == 'system'
+        assert 'Current date:' in messages[0].content
+
+    @pytest.mark.asyncio
+    async def test_non_local_agent_runner_skips_date_injection(self):
+        """Runners other than local-agent should not get the date addition."""
+        preproc = get_preproc_module()
+
+        app = FakeApp()
+        mock_session = make_session()
+        app.sess_mgr.get_session = AsyncMock(return_value=mock_session)
+
+        mock_conversation = Mock()
+        mock_conversation.prompt = Mock(messages=[])
+        mock_conversation.prompt.copy = Mock(return_value=Mock(messages=[]))
+        mock_conversation.messages = []
+        mock_conversation.uuid = None
+        app.sess_mgr.get_conversation = AsyncMock(return_value=mock_conversation)
+
+        app.model_mgr.get_model_by_uuid = AsyncMock(return_value=None)
+        app.tool_mgr.get_all_tools = AsyncMock(return_value=[])
+
+        mock_event_ctx = Mock()
+        mock_event_ctx.event = Mock(default_prompt=[], prompt=[])
+        app.plugin_connector.emit_event = AsyncMock(return_value=mock_event_ctx)
+
+        stage = preproc.PreProcessor(app)
+        query = text_query('hello')
+        query.pipeline_config = {
+            'ai': {
+                'runner': {'runner': 'dify-service-api'},
+                'local-agent': {'model': {'primary': '', 'fallbacks': []}, 'prompt': 'default'},
+            },
+            'output': {'misc': {'at-sender': False}},
+            'trigger': {'misc': {}},
+        }
+
+        result = await stage.process(query, 'PreProcessor')
+
+        assert result.new_query.prompt.messages == []