fix(config): align prefill messages key handling

tests/cli/test_prefill_config.py

@@ -0,0 +1,35 @@
+"""Regression tests for CLI prefill config key compatibility."""
+
+from __future__ import annotations
+
+import cli
+
+
+def test_resolve_prefill_messages_file_uses_top_level(monkeypatch):
+    monkeypatch.delenv("HERMES_PREFILL_MESSAGES_FILE", raising=False)
+
+    assert cli._resolve_prefill_messages_file(
+        {
+            "prefill_messages_file": "top.json",
+            "agent": {"prefill_messages_file": "legacy.json"},
+        }
+    ) == "top.json"
+
+
+def test_resolve_prefill_messages_file_accepts_legacy_agent_key(monkeypatch):
+    monkeypatch.delenv("HERMES_PREFILL_MESSAGES_FILE", raising=False)
+
+    assert cli._resolve_prefill_messages_file(
+        {"agent": {"prefill_messages_file": "legacy.json"}}
+    ) == "legacy.json"
+
+
+def test_resolve_prefill_messages_file_prefers_env(monkeypatch):
+    monkeypatch.setenv("HERMES_PREFILL_MESSAGES_FILE", "env.json")
+
+    assert cli._resolve_prefill_messages_file(
+        {
+            "prefill_messages_file": "top.json",
+            "agent": {"prefill_messages_file": "legacy.json"},
+        }
+    ) == "env.json"