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/services/database.service.ts

@@ -270,6 +270,9 @@ export class DatabaseService {
       { key: 'logging.auditEnabled', value: 'true', category: 'logging' },
       { key: 'logging.traceEnabled', value: 'true', category: 'logging' },
 
+      // Ollama 配置
+      { key: 'ollama.numCtx', value: 'null', category: 'ollama' },
+
       // Onboarding
       { key: 'onboarding.completed', value: 'false', category: 'general' },
     ];