On Windows, prompt_toolkit produces lone surrogate code points (e.g.
🐈) for emoji input. These propagate through the message bus
and crash at json.dumps() / file write time because surrogates cannot
be encoded as UTF-8.
Extract _sanitize_surrogates() that round-trips through UTF-16 to
reconstruct paired surrogates into real characters (e.g. 🐈
→ 🐈), replacing unpaired surrogates with U+FFFD. Apply it at the CLI
input path and reuse in SafeFileHistory.
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