fix: _witness_role_for defensive None handling (T79)

chat/services/prompt.py

@@ -379,8 +379,15 @@ def _witness_role_for(speaker_bot_id: str, host_bot_id: str | None) -> str:
     pinned the contract on ``search_memories``; this helper applies it
     at the call site so a guest-as-speaker doesn't silently retrieve
     memories under the wrong POV mask.
+
+    When ``host_bot_id`` is ``None`` (degenerate case from a half-seeded
+    chat or Phase-1 path), the speaker is treated as the host so the
+    query falls back to the host POV mask rather than silently masking
+    the speaker's own memories as a guest.
     """
-    return "host" if speaker_bot_id == host_bot_id else "guest"
+    if host_bot_id is None or speaker_bot_id == host_bot_id:
+        return "host"
+    return "guest"
 
 
 def _resolve_addressee(