docs: add AI-focused v13 → v14 migration guide

Adds ai-docs/ai-migration.md, a token-optimized execution guide for coding
agents performing the v13 → v14 migration. Distills the 62 confirmed
breaking changes from ai-docs/breaking-changes.md into ordered phases,
rename lists, and code-level rewrites.

Removes the superseded ai-docs/DEPRECATED_API_REMOVAL_PLAN.md.

Author: oliverlaz