[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- Explicit notes for historical proposals that were retained, superseded, deferred, or moved elsewhere.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.

Start by reviewing the final decisions and implementations in the other v2 sub-issues, then compare them with the shipped preview packages. Build the per-language inventory, examples, compatibility notes, replacements, and upgrade instructions, linking decision issues and implementation pull requests. Done means each affected SDK owner has reviewed it and the guide is linked from the v2 release notes and primary documentation.

Written by the indexing model from the issue text.