feat(core): 实现 Doom Loop 检测和软性 maxSteps 限制

- 新增 doom-loop.ts: 实现 Doom Loop 检测器，检测连续 3 次相同工具调用
- 修改 agent.ts: 集成检测器，添加 onDoomLoop 回调，动态 maxSteps
- 修改 registry.ts: 默认 maxSteps 从 10 改为 50
- 更新 index.ts: 导出新模块

参考 OpenCode/OpenHands 等开源项目的多层防御策略

packages/core/src/index.ts

@@ -1,5 +1,14 @@
 export { Agent } from './core/agent.js';
-export type { AgentChatOptions, ToolStartInfo, ToolEndInfo } from './core/agent.js';
+export type { AgentChatOptions, ToolStartInfo, ToolEndInfo, DoomLoopInfo } from './core/agent.js';
+
+// Doom Loop Detection
+export {
+  createDoomLoopDetector,
+  DOOM_LOOP_THRESHOLD,
+  DOOM_LOOP_WARNING,
+  MAX_STEPS_WARNING,
+} from './core/doom-loop.js';
+export type { DoomLoopDetector } from './core/doom-loop.js';
 export { toolRegistry, todoManager, initTaskContext, updateTaskDescription, updateSkillDescription } from './tools/index.js';
 export { loadConfig, saveConfig, getConfig, loadVisionConfig, ConfigurationError } from './utils/config.js';
 export type { VisionConfig } from './utils/config.js';