docs: document memory backends

[Viperdk2020]

Summary

• explain JSONL vs SQLite memory backends
• mention sqlite env var and migrate/compact commands
• reference new memory backend guide from README and config docs

Testing

• pnpm format (fails: AGENTS.md not formatted)

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https://chatgpt.com/codex/tasks/task_e_68b515d3985883298647153ea52914c6

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