[v2] Publish the breaking-change inventory and migration guide

## Summary

Publish the authoritative breaking-change inventory and migration guidance for Copilot SDK v2.

This work depends on the final decisions and implementations in the other v2 sub-issues.

## Required content

- Per-language inventory of removed, renamed, and relocated APIs.
- Before-and-after examples for every accepted breaking change.
- Behavioral changes that may compile successfully but alter runtime behavior.
- Supported replacements for removed APIs.
- Runtime and CLI compatibility requirements.
- Package/version upgrade instructions.
- Links to the decision issues and implementation pull requests.
- Explicit notes for historical proposals that were retained, superseded, deferred, or moved elsewhere.

## Completion

The migration guide is reviewed by each affected SDK owner, matches the shipped preview packages, and is linked from the v2 release notes and primary documentation.

先审查其他 v2 子 issue 中的最终决策和实现，然后将其与已发布的预览包进行比较。构建按语言划分的清单、示例、兼容性说明、替代项和升级说明，并链接决策 issue 和实现 pull request。所有受影响的 SDK 负责人都完成审查，并且该指南已从 v2 发布说明和主要文档中链接时，才算完成。

由索引模型根据 Issue 内容生成。