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_hook_composite.py

@@ -6,6 +6,7 @@ from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
+from nanobot.agent.context import TranscriptInput
 from nanobot.agent.hook import (
     AgentHook,
     AgentHookContext,
@@ -459,7 +460,7 @@ async def test_agent_loop_extra_hook_receives_calls(tmp_path):
     loop.tools.get_definitions = MagicMock(return_value=[])
 
     result = await loop._run_agent_loop(
-        [{"role": "user", "content": "hi"}],
+        TranscriptInput(history=[{"role": "user", "content": "hi"}], current_message=None),
         runtime=loop.llm_runtime(),
     )
 
@@ -504,7 +505,7 @@ async def test_agent_loop_turn_hook_factories_receive_context(tmp_path):
 
     runtime = loop.llm_runtime()
     await loop._run_agent_loop(
-        [{"role": "user", "content": "hi"}],
+        TranscriptInput(history=[{"role": "user", "content": "hi"}], current_message=None),
         runtime=runtime,
         on_progress=on_progress,
         request_context=RequestContext(
@@ -551,7 +552,7 @@ async def test_agent_loop_extra_hook_error_isolation(tmp_path):
     loop.tools.get_definitions = MagicMock(return_value=[])
 
     result = await loop._run_agent_loop(
-        [{"role": "user", "content": "hi"}],
+        TranscriptInput(history=[{"role": "user", "content": "hi"}], current_message=None),
         runtime=loop.llm_runtime(),
     )
 
@@ -577,7 +578,9 @@ async def test_agent_loop_extra_hooks_do_not_swallow_loop_hook_errors(tmp_path):
 
     with pytest.raises(RuntimeError, match="progress failed"):
         await loop._run_agent_loop(
-            [], runtime=loop.llm_runtime(), on_progress=bad_progress
+            TranscriptInput(history=[], current_message=None),
+            runtime=loop.llm_runtime(),
+            on_progress=bad_progress,
         )
 
 
@@ -596,7 +599,8 @@ async def test_agent_loop_no_hooks_backward_compat(tmp_path):
     loop.max_iterations = 2
 
     result = await loop._run_agent_loop(
-        [], runtime=loop.llm_runtime()
+        TranscriptInput(history=[], current_message=None),
+        runtime=loop.llm_runtime(),
     )
     assert result.final_content == (
         "I reached the maximum number of tool call iterations (2) "