docs(hermes): plan context engine support

[RerankerGuo]

Description | 描述

Add a design plan for bringing TencentDB Agent Memory short-term context offload to Hermes through a ContextEngine plugin.

The document summarizes:

• the Hermes ContextEngine lifecycle contract
• how it maps to memory-tencentdb short-term offload and Mermaid task canvases
• a phased implementation plan from no-op skeleton to read-only MMD injection, offload write path, and lookup tools
• proposed config separation from the existing Hermes memory.provider
• validation requirements before enabling runtime behavior

This is intentionally design-only. It does not claim to complete Hermes Context Engine support.

Related Issue | 关联 Issue

Related to #102
Related to #202

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

Verified with npm test and npm run build using Node v24.15.0.

[Maxwell-Code07]

Thanks for your attention. We will review it and get back to you soon.