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/agent/test_attachment_references.py

@@ -5,6 +5,7 @@ from unittest.mock import AsyncMock, MagicMock
 
 import pytest
 
+from nanobot.agent.context import TranscriptInput
 from nanobot.agent.loop import AgentLoop, TurnContext, TurnKind
 from nanobot.agent.tools.context import RequestContext
 from nanobot.agent.tools.filesystem import ReadFileTool
@@ -148,7 +149,10 @@ async def test_pending_document_attachment_keeps_body_out_of_prompt(
 
     runtime = loop.llm_runtime()
     result = await loop._run_agent_loop(
-        [{"role": "user", "content": "hello"}],
+        TranscriptInput(
+            history=[{"role": "user", "content": "hello"}],
+            current_message=None,
+        ),
         runtime=runtime,
         request_context=RequestContext(channel="cli", chat_id="c", runtime=runtime),
         pending_queue=pending_queue,