feat: zkStash langchainjs integration

[chentschel]

Introducing @langchain/zkstash

This PR introduces the @langchain/zkstash provider, a managed Long-Term Memory (LTM) for autonomous agents.

Key Components

ZkStashMemoryMiddleware: A comprehensive Agent Middleware that automates the LTM lifecycle:

• Performs sliding-window semantic search and injects relevant context before the LLM call.
• Automatically extracts new structured knowledge from Human-AI interaction pairs and saves it to the agent's brain.
• Uses a configurable search context window to provide semantic depth for follow-up user queries (e.g., "Why?").
• History Hygiene: Implements ID-based message replacement (RemoveMessage) to prevent conversation history bloat.

ZkStashRetriever: A standard BaseRetriever implementation for manual or agentic semantic search across zkStash memories.

ZkStashStore: A BaseStore implementation. By mapping keys to zkStash schemas, agents can use to manage their complex, persistent knowledge base.

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