chore: remove defensive stale-guest degrade in turns.py (T74.4)

T44 carried a defensive degrade-to-1:1 block in post_turn for the
case where chat.guest_bot_id pointed at a deleted bot. T47 then
fixed the root cause by adding a bot_reset cascade that clears
guest_bot_id from any chat that referenced the deleted bot, so the
post_turn defensive block was rendered dead.

Remove the orphan-clear branch and replace it with a comment
documenting that get_bot now returns a real row when guest_bot_id
is non-None. The cascade behavior is pinned by
test_reset_clears_guest_reference_in_other_chats in tests/test_reset.py.

chat/web/turns.py

@@ -236,11 +236,12 @@ async def post_turn(
     guest_bot = None
     guest_bot_id = chat.get("guest_bot_id")
     if guest_bot_id is not None:
+        # T47's bot_reset cascade clears guest_bot_id from any chat that
+        # referenced the deleted bot, so by the time we read it here it's
+        # either None or a live bot id. The previous defensive
+        # degrade-to-1:1 block (T44) was rendered dead by T47 and removed
+        # in T74.4 — get_bot now returns a real row.
         guest_bot = get_bot(conn, guest_bot_id)
-        # If the chat references a deleted guest we degrade to single-bot
-        # rather than 404 — the chat is still usable as a 1:1.
-        if guest_bot is None:
-            guest_bot_id = None
 
     settings = request.app.state.settings