fix(config): write config.json atomically via temp+replace

save_config truncated config.json in place on crash mid-write.
Route through _write_text_atomic like the pairing store so a failed write leaves the prior file intact.

tests/config/test_config_atomic_save.py

@@ -0,0 +1,36 @@
+"""Tests for atomic config.json writes."""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import pytest
+
+from nanobot.config.loader import load_config, save_config
+from nanobot.config.schema import Config
+
+
+def test_save_config_round_trips(tmp_path: Path) -> None:
+    path = tmp_path / "config.json"
+    save_config(Config(), path)
+    loaded = load_config(path)
+    assert loaded.agents.defaults.model
+
+
+def test_save_config_preserves_existing_file_when_write_fails(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    path = tmp_path / "config.json"
+    save_config(Config(), path)
+    before = path.read_text(encoding="utf-8")
+
+    def boom(self: Path, target: Path) -> Path:  # noqa: ARG001
+        raise OSError("simulated crash before replace")
+
+    monkeypatch.setattr(Path, "replace", boom)
+    with pytest.raises(OSError, match="simulated crash"):
+        save_config(Config(), path)
+
+    assert path.read_text(encoding="utf-8") == before
+    assert json.loads(before)  # still valid JSON