Incremental state persistence with JSONL format

[krasserm]

Summary

• Migrate DataStore from JSON snapshots to append-only JSONL format
• Replace get_serialized()/set_serialized() with get_new_messages()/set_serialized(lines) for incremental persistence
• State is now persisted after each run() call instead of on shutdown

Test plan

• All existing tests pass (45 tests)
• New tests for JSONL-specific behavior (malformed JSON, empty lines, empty list append)

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