[Frontend Feature] Transform agent interface to plugin/independent sdk

**What would you like to be added**:
Agent 前端插件化，留出上下文的一些扩展接口

**Why is this needed**:

The issue does not name any files, tests, entry points, or specific extension points. Start by locating the agent frontend interface and clarifying the plugin and independent SDK boundaries, required context APIs, and acceptance criteria. Done should include an agreed design and verifiable behavior for the proposed extension interfaces.

Written by the indexing model from the issue text.