Persist JsonlMemoryStore usage counters

[Viperdk2020]

Summary

• maintain file path in JsonlMemoryStore
• update used_count and last_used_at during recall
• add regression test for counter persistence

Testing

• cargo fmt -p codex-core
• cargo clippy -p codex-core --fix --allow-dirty --allow-staged (failed: terminated due to timeout)
• cargo test -p codex-core (failed: terminated early while running integration tests)

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https://chatgpt.com/codex/tasks/task_e_68b5153f2e9c8329acc3e280b2ade95a

[github-actions]

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