feat: add plugin system for cortex and storage adapters

Simple plugin architecture with two use cases:
1. Native acceleration (@soulcraft/brainy-cortex) — auto-detected
2. Custom storage adapters (e.g., Redis, DynamoDB)

Plugins implement BrainyPlugin interface with activate/deactivate lifecycle.
Auto-detection tries dynamic import of known packages during init().
Manual registration via brain.use(plugin) before init().

Provider keys: metadataIndex, graphIndex, entityIdMapper, cache, hnsw,
roaring, embeddings, distance, msgpack, storage:<name>

src/index.ts

@@ -109,6 +109,10 @@ export {
 // Export version utilities
 export { getBrainyVersion } from './utils/version.js'
 
+// Export plugin system
+export type { BrainyPlugin, BrainyPluginContext, StorageAdapterFactory } from './plugin.js'
+export { PluginRegistry } from './plugin.js'
+
 // Export embedding functionality
 import {
   UniversalSentenceEncoder,