feat: add migration system with error handling, validation, and enterprise hardening
- MigrationRunner: per-entity error tracking (non-fatal), maxErrors bail-out,
static validateMigrations() called in constructor, branch error propagation
- RefManager: updateRefMetadata() method for clean metadata updates
- brainy.ts: eliminate as-any casts in migration methods, use updateRefMetadata,
forward maxErrors through full chain including branch migrations
- Types: MigrationError interface, errors field on MigrationResult, maxErrors on MigrateOptions
- Package exports: MigrationError type, migrate() on BrainyInterface, autoMigrate config
- 31 integration tests covering error handling, validation, branch error propagation
- Documentation: docs/guides/schema-migrations.md
David Snelling
39b099cafc