优化: df-ai/df-storage/df-nodes 底层加固: 上下文/模型/节点/仓储

df-ai:
- context/mod.rs + manager_tests.rs: 上下文管理调整 + 测试同步(48 行)
- openai_compat.rs: OpenAI 兼容层增强(+51)
- presets/models.json: 模型预设 +18

df-storage:
- crud/conversation_repo.rs: 会话仓储(loop_state 相关)
- crud/task_repo.rs: 任务仓储
- crud/mod.rs: 导出调整

df-nodes:
- task_advance_node.rs: 任务推进节点

验收: cargo check -p devflow EXIT 0。

crates/df-ai/presets/models.json

@@ -136,6 +136,24 @@
     "context_window": 262144,
     "temperature_supported": false
   },
+  {
+    "model_id": "sensenova-6.8-flash-lite",
+    "enabled": true,
+    "modalities": ["text", "vision"],
+    "capabilities": ["tool_use"],
+    "weight": 85,
+    "context_window": 262144,
+    "temperature_supported": false
+  },
+  {
+    "model_id": "sn2-kimi",
+    "enabled": true,
+    "modalities": ["text"],
+    "capabilities": ["tool_use"],
+    "weight": 80,
+    "context_window": 131072,
+    "temperature_supported": false
+  },
   {
     "model_id": "deepseek-v4-flash",
     "enabled": true,