feat(usage): add unified provider usage backend

tests/agent/test_subagent.py

@@ -1,5 +1,6 @@
 """Tests for SubagentManager."""
 
+import asyncio
 from pathlib import Path
 from unittest.mock import AsyncMock, MagicMock
 
@@ -10,6 +11,7 @@ from nanobot.agent.subagent import SubagentManager, SubagentStatus
 from nanobot.agent.tools.filesystem import FileToolsConfig
 from nanobot.bus.queue import MessageBus
 from nanobot.config.schema import ToolsConfig
+from nanobot.llm_usage.context import llm_usage_source
 from nanobot.providers.base import GenerationSettings, LLMProvider
 from nanobot.security.workspace_access import build_workspace_scope
 from nanobot.utils.llm_runtime import LLMRuntime
@@ -198,3 +200,30 @@ async def test_subagent_forwards_fail_on_tool_error_to_runner(tmp_path):
 
     spec = sm.runner.run.call_args.args[0]
     assert spec.fail_on_tool_error is False
+
+
+@pytest.mark.asyncio
+async def test_spawned_subagent_inherits_llm_usage_source(tmp_path):
+    provider = MagicMock(spec=LLMProvider)
+    provider.get_default_model.return_value = "test"
+    sm = SubagentManager(
+        workspace=tmp_path,
+        bus=MessageBus(),
+        max_tool_result_chars=16_000,
+    )
+    sm.runner.run = AsyncMock(
+        return_value=AgentRunResult(final_content="ok", messages=[], stop_reason="completed")
+    )
+    sm._announce_result = AsyncMock()
+
+    with llm_usage_source("cron"):
+        await sm.spawn(
+            "automation task",
+            session_key="websocket:bound-automation",
+            runtime=_runtime(provider),
+        )
+    tasks = list(sm._running_tasks.values())
+    await asyncio.gather(*tasks)
+
+    spec = sm.runner.run.call_args.args[0]
+    assert spec.llm_usage_source == "cron"