docs: add memory backend guide

[Viperdk2020]

Summary

• explain JSONL vs SQLite memory stores
• show how to enable SQLite and migrate or compact existing data
• link memory backend guide from README and config docs

Testing

• pnpm exec prettier --check README.md docs/config.md docs/memory-backends.md

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https://chatgpt.com/codex/tasks/task_e_68b510aa241083298a4579bc7355e434

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