refactor(/new): background archival with guaranteed persistence

Replace fire-and-forget consolidation with archive_messages(), which
retries until the raw-dump fallback triggers — making it effectively
infallible. /new now clears the session immediately and archives in
the background. Pending archive tasks are drained on shutdown via
close_mcp() so no data is lost on process exit.

nanobot/agent/memory.py

@@ -290,14 +290,14 @@ class MemoryConsolidator:
             self._get_tool_definitions(),
         )
 
-    async def archive_unconsolidated(self, session: Session) -> bool:
-        """Archive the full unconsolidated tail for /new-style session rollover."""
-        lock = self.get_lock(session.key)
-        async with lock:
-            snapshot = session.messages[session.last_consolidated:]
-            if not snapshot:
+    async def archive_messages(self, messages: list[dict[str, object]]) -> bool:
+        """Archive messages with guaranteed persistence (retries until raw-dump fallback)."""
+        if not messages:
+            return True
+        for _ in range(self.store._MAX_FAILURES_BEFORE_RAW_ARCHIVE):
+            if await self.consolidate_messages(messages):
                 return True
-            return await self.consolidate_messages(snapshot)
+        return True
 
     async def maybe_consolidate_by_tokens(self, session: Session) -> None:
         """Loop: archive old messages until prompt fits within half the context window."""