MigraDiff v1.3.0 adds AI-powered migration explanations and migrations folder input mode

MigraDiff released version 1.3.0 with an `--explain` flag that uses Claude Haiku (Anthropic's LLM) to describe PostgreSQL schema migrations in plain English, including risks and safer alternatives for destructive operations. Users provide their own API key; no data is sent to MigraDiff servers.

New `--from-migrations-dir` mode allows diffing a directory of numbered migration files against a base schema without a live branch database. Supports Supabase timestamp format, Flyway versioned format, and standard numeric prefixes, applying files in numeric sort order.

The explanation feature works with existing flags (`--output json`, `--from-file`, `--from-migrations-dir`) and is installed via `pip install migradiff[ai]`. The package name is now `migradiff` while the CLI command remains `migra` for backward compatibility.