feat: Agnes上下文512K→1M + Ollama支持配置num_ctx

Agnes AI:
- adapter注释和API文档更新: 上下文512K→1M (官方已升级)

Ollama num_ctx 支持:
- MetonaGenerationParams 新增 contextLength 字段
- OllamaAdapter.toNativeRequest 发送 options.num_ctx
- AgentLoopEngine 从 config 读取并传递到请求参数
- database.service 新增 ollama.numCtx 默认配置(null=由模型决定)
- SettingsModal LLM设置: Ollama 选中时显示上下文长度输入框
- main.ts 启动时从 configService 读取 ollama.numCtx

数据流:
Settings → configService → main.ts → AgentLoopEngine.config
→ MetonaRequest.params → OllamaAdapter → options.num_ctx

electron/main.ts

@@ -147,7 +147,11 @@ async function initialize(): Promise<void> {
   ];
 
   // ===== Agent Loop =====
-  const agentLoop = new AgentLoopEngine({}, adapter, toolRegistry, preToolHooks, postToolHooks);
+  const ollamaNumCtx = configService.get<number>('ollama.numCtx');
+  const agentLoop = new AgentLoopEngine(
+    { contextLength: ollamaNumCtx ?? undefined },
+    adapter, toolRegistry, preToolHooks, postToolHooks,
+  );
   agentLoop.setTools(toolRegistry.listTools());
   agentLoop.setWorkspacePath(workspaceInfo.path);