feat(usage): add unified provider usage backend

tests/llm_usage/test_llm_usage_context.py

@@ -0,0 +1,19 @@
+from nanobot.llm_usage.context import source_from_request
+
+
+def test_automation_metadata_overrides_user_session_source() -> None:
+    assert source_from_request(
+        "websocket:ordinary-session",
+        channel="websocket",
+        metadata={"_cron_trigger": {"job_id": "job"}},
+    ) == "cron"
+    assert source_from_request(
+        "websocket:ordinary-session",
+        channel="websocket",
+        metadata={"_local_trigger": {"trigger_id": "trigger"}},
+    ) == "cron"
+
+
+def test_api_and_system_channels_have_explicit_sources() -> None:
+    assert source_from_request("shared-session", channel="api", metadata={}) == "api"
+    assert source_from_request("shared-session", channel="system", metadata={}) == "system"