feat(core): seeds without text wait for the first turn

tests/test_conversations.py

@@ -396,8 +396,14 @@ async def test_copy_seed_forks_parent_with_window(world: World) -> None:
         "q6",
         [{"type": "text", "text": "a7"}],
     ]
+    assert (await world.conversations.get(child.external_id)).flags["seed"] == "copy"
+    await world.conversations.post(child, "go")
     await world.settle(child, 1)
     assert ScriptedClient.instances[0].options.resume == child.session_id
+    prompt = ScriptedClient.instances[0].prompts[0]
+    assert prompt.startswith("[сид: copy] branch, ")
+    assert "последние 1 тёрнов" in prompt and prompt.endswith("\n\ngo")
+    assert (await world.conversations.get(child.external_id)).flags["seed"] is None
 
 
 async def test_merge_injects_summary_into_parent(world: World) -> None: