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/types/brainy.types.ts

@@ -837,6 +837,11 @@ export interface BrainyConfig {
   // - false/undefined: Disable integrations (default)
   // - IntegrationsConfig: Custom configuration
   integrations?: boolean | IntegrationsConfig
+
+  // Migration configuration
+  // - false/undefined (default): Log warning if pending migrations exist, but don't auto-run
+  // - true: Automatically run pending migrations during init() for small datasets (<10K entities)
+  autoMigrate?: boolean
 }
 
 // ============= Neural API Types =============