fix: make Butler runtime probes reliable

tests/test_app.py

@@ -42,7 +42,7 @@ def test_health_exposes_current_version():
     with TestClient(app.app) as client:
         response = client.get("/health")
     assert response.status_code == 200
-    assert response.json()["version"] == app.VERSION == "2.3.0"
+    assert response.json()["version"] == app.VERSION == "2.3.1"
 
 
 def test_invalid_log_target_is_rejected_before_ssh():
@@ -145,6 +145,24 @@ def test_backup_item_reports_age_and_severity():
     assert app._backup_item(1, "", "timeout")["state"] == "unknown"
 
 
+def test_ssh_uses_writable_runtime_known_hosts(monkeypatch):
+    captured = {}
+
+    class Result:
+        returncode = 0
+        stdout = "ok"
+        stderr = ""
+
+    def fake_run(args, **_kwargs):
+        captured["args"] = args
+        return Result()
+
+    monkeypatch.setattr(app._sp, "run", fake_run)
+    assert app._ssh("sascha@example", "true", timeout=1)[0] == 0
+    joined = " ".join(captured["args"])
+    assert "UserKnownHostsFile=/tmp/butler_known_hosts" in joined
+
+
 def test_ssh_timeout_is_normalized_instead_of_crashing_collection(monkeypatch):
     def timeout(*_args, **_kwargs):
         raise app._sp.TimeoutExpired(cmd=["ssh"], timeout=1)