feat: Add files for major AI tools, point all to AGENTS.md

[royanger]

This branch is all about making the migration tool easier for AI agents (Claude, Copilot, Cursor, etc.) to work with, plus
some general quality-of-life improvements.

• AI agent support — Added AGENTS.md as the single source of truth for how the codebase works, then pointed all the AI config files (CLAUDE.md, CODEX.md, .cursorrules, copilot-instructions, etc.) at it. Also added Claude skills for running migrations and generating transformers.
• Non-interactive CLI mode — The migrate script can now be run with CLI flags (--transformer, --file, --clerk-secret-key, etc.) so agents don't have to navigate interactive prompts. Includes --help and a --non-interactive mode.
• Docs cleanup — Split the big README into focused docs (schema-fields.md, creating transformers.md) and added dedicated AI prompts for migration and transformer creation.
• Code reorg — Moved transformers from src/migrate/transformers/ up to src/transformers/ since they're used more broadly.
• Consolidated types into src/types.ts.
• Tests & misc — Added transformer tests, expanded delete tests, added banned field support, improved Firebase transformer, and added a test setup file.