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_loop_tool_context.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
 from nanobot.agent.tools.context import (
     RequestContext,
@@ -133,7 +134,7 @@ async def test_loop_binds_request_context_for_tool_execution(tmp_path: Path) ->
     metadata = {"slack": {"thread_ts": "111.222", "channel_type": "channel"}}
     runtime = loop.llm_runtime()
     await loop._run_agent_loop(
-        [],
+        TranscriptInput(history=[], current_message=None),
         runtime=runtime,
         request_context=RequestContext(
             channel="slack",
@@ -234,7 +235,7 @@ async def test_agent_loop_restores_outer_request_context_after_runner_exception(
     try:
         with pytest.raises(RuntimeError, match="runner failed"):
             await loop._run_agent_loop(
-                [],
+                TranscriptInput(history=[], current_message=None),
                 runtime=runtime,
                 request_context=RequestContext(
                     channel="slack",