docs(ai): add v6 to v7 AI migration guide

Machine-oriented migration guide for AI agents migrating Android Chat
SDK from v6 to v7. Covers dependencies, plugin system removal, theming
redesign, package relocations, component renames, and verification.

Tested by having an AI agent migrate the v6 compose sample app to v7
with zero prior SDK knowledge — builds successfully.

Author: aleksandar-apostolov