fix: simplify session recency activity tracking

maintainer edit: remove the _last_compacted_at maintenance state, gate idle compaction on whether a session still has a removable tail, and sort WebUI sessions by the latest visible transcript activity.

tests/agent/test_autocompact_unit.py

@@ -6,7 +6,6 @@ from unittest.mock import AsyncMock, MagicMock
 import pytest
 
 from nanobot.agent.autocompact import AutoCompact
-from nanobot.agent.memory import LAST_COMPACTED_AT_META
 from nanobot.session.manager import Session, SessionManager
 
 
@@ -201,8 +200,11 @@ class TestCheckExpired:
         """Expired session should trigger schedule_background."""
         ac = _make_autocompact(ttl=15)
         mock_sm = MagicMock(spec=SessionManager)
-        old_ts = (datetime.now() - timedelta(minutes=20)).isoformat()
-        mock_sm.list_sessions.return_value = [{"key": "cli:old", "updated_at": old_ts}]
+        old_dt = datetime.now() - timedelta(minutes=20)
+        session = _make_session("cli:old", updated_at=old_dt)
+        _add_turns(session, 5)
+        mock_sm.list_sessions.return_value = [{"key": "cli:old", "updated_at": old_dt.isoformat()}]
+        mock_sm.get_or_create.return_value = session
         ac.sessions = mock_sm
 
         scheduled = []
@@ -274,17 +276,17 @@ class TestCheckExpired:
         scheduler.assert_not_called()
         assert "dream:20260602-155256" not in ac._archiving
 
-    def test_already_compacted_session_skips(self):
-        """Expired session already maintained after last activity should not be re-scheduled."""
+    def test_already_trimmed_session_skips(self):
+        """Expired session with no removable tail should not be re-scheduled."""
         ac = _make_autocompact(ttl=15)
         mock_sm = MagicMock(spec=SessionManager)
         last_active = datetime(2026, 1, 1, 10, 0, 0)
+        session = _make_session("cli:done", updated_at=last_active)
+        _add_turns(session, 2)
         mock_sm.list_sessions.return_value = [
             {"key": "cli:done", "updated_at": last_active.isoformat()},
         ]
-        mock_sm.read_session_metadata.return_value = {
-            "metadata": {LAST_COMPACTED_AT_META: datetime(2026, 1, 1, 10, 30, 0).isoformat()},
-        }
+        mock_sm.get_or_create.return_value = session
         ac.sessions = mock_sm
 
         scheduler = MagicMock()