feat(core): add optional pipeline stage controls

[Dimon94]

Description | 描述

Draft implementation for optional downstream pipeline controls discussed in #26:

• add pipeline.enableL2 and pipeline.enableL3, both defaulting to true
• preserve current OpenClaw behavior when the options are unset
• skip L2/L3 scheduling when disabled
• avoid creating the tool-enabled L2/L3 runner when neither downstream stage is needed
• document the new config fields in the English and Chinese config tables

This is opened as a draft because it is a product/behavior choice. The implementation is intentionally small so maintainers can first decide whether this control belongs in the project.

Related Issue | 关联 Issue

Discusses #26.

Change Type | 修改类型

• Bug fix | Bug 修复
• New feature | 新功能
• Documentation update | 文档更新
• Code optimization | 代码优化

Self-test Checklist | 自测清单

• Verified locally | 本地验证通过
• No existing features affected | 无影响现有功能

Additional Notes | 其他说明

Local verification:

• npm test -- src/core/pipeline-controls.test.ts src/utils/pipeline-manager.test.ts
• npm run build:plugin
• git diff --cached --check before commit