feat(consolidator): add compact_idle_session method with lock-protected truncation

Add Consolidator.compact_idle_session(session_key, max_suffix=8) that
performs hard-truncation of idle sessions under the per-session
consolidation lock. This is the single lock-protected path for AutoCompact
to use instead of modifying session state directly, fixing the race
condition between AutoCompact and Consolidator.

Behavior:
- Acquires per-session consolidation lock
- Invalidates cache and reloads fresh from disk
- Splits unconsolidated tail into archive prefix and retained suffix
- Archives prefix via LLM (with raw_archive fallback on failure)
- Persists _last_summary in session metadata on success
- Returns summary text, None on LLM failure, or '' if nothing to archive

Tests: 6 new tests covering prefix archival, empty session timestamp
refresh, (nothing) summary exclusion, LLM failure fallback,
last_consolidated offset, and lock acquisition verification.

nanobot/agent/memory.py

@@ -769,6 +769,74 @@ class Consolidator:
             # the summary injection strategy with AutoCompact._archive().
             self._persist_last_summary(session, last_summary)
 
+    async def compact_idle_session(
+        self,
+        session_key: str,
+        max_suffix: int = 8,
+    ) -> str | None:
+        """Hard-truncate an idle session under the consolidation lock.
+
+        Used by AutoCompact so all session mutation goes through a single
+        lock-protected path.  Returns the summary text on success, ``None``
+        if the LLM failed (raw_archive fallback), or ``""`` if there was
+        nothing to archive.
+        """
+        lock = self.get_lock(session_key)
+        async with lock:
+            self.sessions.invalidate(session_key)
+            session = self.sessions.get_or_create(session_key)
+
+            tail = list(session.messages[session.last_consolidated:])
+            if not tail:
+                session.updated_at = datetime.now()
+                self.sessions.save(session)
+                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(max_suffix)
+            kept = probe.messages
+            cut = len(tail) - len(kept)
+            archive_msgs = tail[:cut]
+
+            if not archive_msgs and not kept:
+                session.updated_at = datetime.now()
+                self.sessions.save(session)
+                return ""
+
+            last_active = session.updated_at
+            summary: str | None = ""
+            if archive_msgs:
+                summary = await self.archive(archive_msgs)
+
+            if summary and summary != "(nothing)":
+                session.metadata["_last_summary"] = {
+                    "text": summary,
+                    "last_active": last_active.isoformat(),
+                }
+
+            session.messages = kept
+            session.last_consolidated = 0
+            session.updated_at = datetime.now()
+            self.sessions.save(session)
+
+            if archive_msgs:
+                logger.info(
+                    "Idle-session compact for {}: archived={}, kept={}, summary={}",
+                    session_key,
+                    len(archive_msgs),
+                    len(kept),
+                    bool(summary),
+                )
+
+            return summary
+
 
 # ---------------------------------------------------------------------------
 # Dream — heavyweight cron-scheduled memory consolidation