fix(cli): sanitize surrogate code points before entering message bus

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.

tests/cli/test_safe_file_history.py

@@ -3,12 +3,40 @@
 Surrogate characters in CLI input must not crash history file writes.
 """
 
-from nanobot.cli.commands import SafeFileHistory
+from nanobot.cli.commands import SafeFileHistory, _sanitize_surrogates
+
+
+class TestSanitizeSurrogates:
+    def test_paired_surrogates_reconstructed(self):
+        """Windows console produces \\ud83d\\udc08 for U+1F408 — must be restored."""
+        result = _sanitize_surrogates("你为什么会用 🐈")
+        assert result == "你为什么会用 🐈"
+
+    def test_lone_surrogates_replaced(self):
+        result = _sanitize_surrogates("hello \udce9 world")
+        assert "\udce9" not in result
+        assert "hello" in result
+        assert "world" in result
+
+    def test_normal_text_unchanged(self):
+        assert _sanitize_surrogates("normal ascii text") == "normal ascii text"
+
+    def test_emoji_already_correct(self):
+        """Properly encoded emoji should pass through unchanged."""
+        assert _sanitize_surrogates("hello 🐈 nanobot") == "hello 🐈 nanobot"
+
+    def test_mixed_unicode_preserved(self):
+        assert _sanitize_surrogates("你好 hello こんにちは 🎉") == "你好 hello こんにちは 🎉"
+
+    def test_multiple_lone_surrogates(self):
+        result = _sanitize_surrogates("\udce9\udcf1\udcff")
+        assert "\udce9" not in result
+        assert "\udcf1" not in result
+        assert "\udcff" not in result
 
 
 class TestSafeFileHistory:
     def test_surrogate_replaced(self, tmp_path):
-        """Surrogate pairs are replaced with U+FFFD, not crash."""
         hist = SafeFileHistory(str(tmp_path / "history"))
         hist.store_string("hello \udce9 world")
         entries = list(hist.load_history_strings())