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/migrations.ts

@@ -0,0 +1,21 @@
+/**
+ * Migration registry
+ *
+ * Ordered array of migrations. Each migration runs exactly once per storage instance.
+ * Add new migrations at the end — order matters.
+ */
+
+import type { Migration } from './types.js'
+
+export const MIGRATIONS: Migration[] = [
+  // Empty for v7.16.0 — framework scaffolded, ready for future use.
+  // Example of a future migration:
+  //
+  // {
+  //   id: '7.17.0-rename-status-field',
+  //   version: '7.17.0',
+  //   description: 'Rename metadata.state to metadata.status',
+  //   applies: 'nouns',
+  //   transform: (m) => 'state' in m ? { ...m, status: m.state, state: undefined } : null
+  // }
+]