refactor(autocompact): delegate _archive to Consolidator.compact_idle_session

Replace AutoCompact._archive() direct session mutation with delegation
to Consolidator.compact_idle_session(). Remove _split_unconsolidated()
method since that logic now lives inside compact_idle_session.

All session mutation for idle compaction now goes through the
Consolidator's lock, eliminating the race condition between
background token consolidation and idle TTL compaction.

Changes:
- autocompact.py: rewrite _archive() to call compact_idle_session,
  remove _split_unconsolidated(), clean up unused imports
- test_autocompact_unit.py: replace TestArchive/TestSplitUnconsolidated
  with TestArchiveDelegates that verifies delegation behavior
- test_auto_compact.py: convert all consolidator.archive mocks to
  consolidator.compact_idle_session mocks via _make_fake_compact helper

nanobot/agent/autocompact.py

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 from collections.abc import Collection
 from datetime import datetime
-from typing import TYPE_CHECKING, Any, Callable, Coroutine
+from typing import TYPE_CHECKING, Callable, Coroutine
 
 from loguru import logger
 
@@ -37,27 +37,6 @@ class AutoCompact:
     def _format_summary(text: str, last_active: datetime) -> str:
         return f"Previous conversation summary (last active {last_active.isoformat()}):\n{text}"
 
-    def _split_unconsolidated(
-        self, session: Session,
-    ) -> tuple[list[dict[str, Any]], list[dict[str, Any]]]:
-        """Split live session tail into archiveable prefix and retained recent suffix."""
-        tail = list(session.messages[session.last_consolidated:])
-        if not tail:
-            return [], []
-
-        probe = Session(
-            key=session.key,
-            messages=tail.copy(),
-            created_at=session.created_at,
-            updated_at=session.updated_at,
-            metadata={},
-            last_consolidated=0,
-        )
-        probe.retain_recent_legal_suffix(self._RECENT_SUFFIX_MESSAGES)
-        kept = probe.messages
-        cut = len(tail) - len(kept)
-        return tail[:cut], kept
-
     def check_expired(self, schedule_background: Callable[[Coroutine], None],
                       active_session_keys: Collection[str] = ()) -> None:
         """Schedule archival for idle sessions, skipping those with in-flight agent tasks."""
@@ -74,33 +53,17 @@ class AutoCompact:
 
     async def _archive(self, key: str) -> None:
         try:
-            self.sessions.invalidate(key)
-            session = self.sessions.get_or_create(key)
-            archive_msgs, kept_msgs = self._split_unconsolidated(session)
-            if not archive_msgs and not kept_msgs:
-                session.updated_at = datetime.now()
-                self.sessions.save(session)
-                return
-
-            last_active = session.updated_at
-            summary = ""
-            if archive_msgs:
-                summary = await self.consolidator.archive(archive_msgs) or ""
+            summary = await self.consolidator.compact_idle_session(
+                key, self._RECENT_SUFFIX_MESSAGES,
+            )
             if summary and summary != "(nothing)":
-                self._summaries[key] = (summary, last_active)
-                session.metadata["_last_summary"] = {"text": summary, "last_active": last_active.isoformat()}
-            session.messages = kept_msgs
-            session.last_consolidated = 0
-            session.updated_at = datetime.now()
-            self.sessions.save(session)
-            if archive_msgs:
-                logger.info(
-                    "Auto-compact: archived {} (archived={}, kept={}, summary={})",
-                    key,
-                    len(archive_msgs),
-                    len(kept_msgs),
-                    bool(summary),
-                )
+                session = self.sessions.get_or_create(key)
+                meta = session.metadata.get("_last_summary")
+                if isinstance(meta, dict):
+                    self._summaries[key] = (
+                        meta["text"],
+                        datetime.fromisoformat(meta["last_active"]),
+                    )
         except Exception:
             logger.exception("Auto-compact: failed for {}", key)
         finally: