refactor(agent): defer transcript assembly to runner (#5608)

* refactor(agent): defer transcript assembly to runner

Keep persisted history and the fresh turn as explicit inputs until the Runner assembles the provider transcript. Preserve ContextBuilder and direct AgentRunner compatibility while making the save boundary structural.

Refs NAN-81.

* fix(providers): preserve mixed adjacent user content

tests/cli/test_restart_command.py

@@ -11,6 +11,7 @@ from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
+from nanobot.agent.context import TranscriptInput
 from nanobot.bus.events import InboundMessage
 from nanobot.providers.base import LLMResponse, LLMUsage
 
@@ -311,10 +312,16 @@ class TestRestartCommand:
             LLMResponse(content="second", usage=None),
         ])
 
-        first = await loop._run_agent_loop([], runtime=loop.llm_runtime())
+        first = await loop._run_agent_loop(
+            TranscriptInput(history=[], current_message=None),
+            runtime=loop.llm_runtime(),
+        )
         assert first.usage == LLMUsage.reported(input_tokens=9, output_tokens=4)
 
-        second = await loop._run_agent_loop([], runtime=loop.llm_runtime())
+        second = await loop._run_agent_loop(
+            TranscriptInput(history=[], current_message=None),
+            runtime=loop.llm_runtime(),
+        )
         assert second.usage == LLMUsage.estimated(input_tokens=123, output_tokens=7)
 
     @pytest.mark.asyncio