fix: _witness_role_for defensive None handling (T79)

tests/test_prompt.py

@@ -21,7 +21,7 @@ import chat.state.world  # noqa: F401
 import chat.state.events  # noqa: F401
 import chat.state.threads  # noqa: F401
 from chat.llm.client import Message
-from chat.services.prompt import assemble_narrative_prompt
+from chat.services.prompt import _witness_role_for, assemble_narrative_prompt
 
 
 def _seed_basic(conn) -> None:
@@ -852,3 +852,10 @@ def test_assemble_with_open_thread_renders_block(tmp_path):
     body = msgs[0].content
     assert "Open threads:" in body
     assert "Maya's job hunt" in body
+
+
+def test_witness_role_for_none_host_returns_host():
+    assert _witness_role_for("bot_a", None) == "host"
+    # Sanity check: existing semantics preserved.
+    assert _witness_role_for("bot_a", "bot_a") == "host"
+    assert _witness_role_for("bot_a", "bot_b") == "guest"