feat: add Chat SDK integration for persistent chat bot memory

[benfrank241]

Summary

• Adds @vectorize-io/hindsight-chat, a wrapper for the Vercel Chat SDK that gives any chat bot long-term memory via Hindsight
• withHindsightChat() wraps onNewMention / onSubscribedMessage handlers with automatic recall, retain, and memory formatting
• Platform-agnostic: works with Slack, Discord, Teams, Google Chat — any Chat SDK adapter
• Follows the same pattern as the CrewAI and Pydantic AI integrations under hindsight-integrations/

What's included

• src/wrapper.ts — withHindsightChat() handler wrapper with auto-recall before handler, optional auto-retain of inbound messages, and manual ctx.retain() / ctx.recall() / ctx.reflect()
• src/format.ts — memoriesAsSystemPrompt() formats recalled memories + entity observations into an LLM system prompt
• src/types.ts — HindsightClient interface, ChatMessage / ChatThread types, all configuration options
• Tests — Unit tests for both wrapper and format utilities

Test plan

• Unit tests pass (vitest)
• Verified end-to-end with a Slack + Discord test bot sharing a single memory bank — memory stored in Slack was recalled in Discord and vice versa

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