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/migration/index.ts

@@ -0,0 +1,7 @@
+/**
+ * Migration system public API
+ */
+
+export { MigrationRunner } from './MigrationRunner.js'
+export { MIGRATIONS } from './migrations.js'
+export type { Migration, MigrationState, MigrationPreview, MigrationResult, MigrateOptions, MigrationError } from './types.js'