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

.gitignore

@@ -31,6 +31,9 @@ coverage/
 # Test results
 tests/results/
 
+# Filesystem test artifacts (created by integration tests)
+test-*/
+
 # IDE files
 .vscode/
 .idea/