feat: add persistent memory system (Phase 2)

Implement file-based persistent memory with read/write/search tools:
- MemoryStore with namespace-scoped JSON storage
- memory-read, memory-write, memory-search builtin tools
- Auto-extraction of facts during context compaction
- Configurable via memory.enabled, memory.dir, memory.max_context_tokens

src/memory/index.ts

@@ -0,0 +1,2 @@
+export { MemoryStore } from './store.js';
+export type { MemoryStoreConfig, SearchResult } from './store.js';