refactor(memory): keep session summaries structured

tests/agent/test_context_builder.py

@@ -1,11 +1,13 @@
 """Tests for ContextBuilder — system prompt and message assembly."""
 
+from datetime import datetime
 from pathlib import Path
 
 import pytest
 
 from nanobot.agent.context import ContextBuilder
 from nanobot.runtime_context import RuntimeContextBlock
+from nanobot.session.summary import SessionSummary
 
 # ---------------------------------------------------------------------------
 # Helpers
@@ -330,14 +332,24 @@ class TestBuildSystemPrompt:
 
     def test_includes_session_summary(self, tmp_path):
         builder = _builder(tmp_path)
-        result = builder.build_system_prompt(session_summary="Previous chat about Python.")
+        result = builder.build_system_prompt(
+            session_summary=SessionSummary(
+                text="Previous chat about Python.",
+                last_active=datetime(2026, 8, 19, 10, 0),
+            )
+        )
         assert "Previous chat about Python." in result
         assert "[Archived Context Summary]" in result
 
     def test_sections_separated_by_separator(self, tmp_path):
         (tmp_path / "AGENTS.md").write_text("Rules.", encoding="utf-8")
         builder = _builder(tmp_path)
-        result = builder.build_system_prompt(session_summary="Summary.")
+        result = builder.build_system_prompt(
+            session_summary=SessionSummary(
+                text="Summary.",
+                last_active=datetime(2026, 8, 19, 10, 0),
+            )
+        )
         assert "\n\n---\n\n" in result
 
     def test_no_bootstrap_no_summary(self, tmp_path):