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

src/index.ts

@@ -106,6 +106,10 @@ export { getBrainyVersion } from './utils/version.js'
 export type { BrainyPlugin, BrainyPluginContext, StorageAdapterFactory } from './plugin.js'
 export { PluginRegistry } from './plugin.js'
 
+// Export migration system
+export { MigrationRunner, MIGRATIONS } from './migration/index.js'
+export type { Migration, MigrationState, MigrationPreview, MigrationResult, MigrateOptions, MigrationError } from './migration/index.js'
+
 // Export embedding functionality
 import {
   UniversalSentenceEncoder,