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.

tests/test_consolidate_offset.py

@@ -506,7 +506,7 @@ class TestNewCommandArchival:
 
     @pytest.mark.asyncio
     async def test_new_clears_session_immediately_even_if_archive_fails(self, tmp_path: Path) -> None:
-        """/new clears session immediately, archive failure only logs warning."""
+        """/new clears session immediately; archive_messages retries until raw dump."""
         from nanobot.bus.events import InboundMessage
 
         loop = self._make_loop(tmp_path)
@@ -516,7 +516,11 @@ class TestNewCommandArchival:
             session.add_message("assistant", f"resp{i}")
         loop.sessions.save(session)
 
+        call_count = 0
+
         async def _failing_consolidate(_messages) -> bool:
+            nonlocal call_count
+            call_count += 1
             return False
 
         loop.memory_consolidator.consolidate_messages = _failing_consolidate  # type: ignore[method-assign]
@@ -524,14 +528,15 @@ class TestNewCommandArchival:
         new_msg = InboundMessage(channel="cli", sender_id="user", chat_id="test", content="/new")
         response = await loop._process_message(new_msg)
 
-        # /new returns immediately with success message
         assert response is not None
         assert "new session started" in response.content.lower()
 
-        # Session is cleared immediately, even though archive will fail in background
         session_after = loop.sessions.get_or_create("cli:test")
         assert len(session_after.messages) == 0
 
+        await loop.close_mcp()
+        assert call_count == 3  # retried up to raw-archive threshold
+
     @pytest.mark.asyncio
     async def test_new_archives_only_unconsolidated_messages(self, tmp_path: Path) -> None:
         from nanobot.bus.events import InboundMessage
@@ -545,12 +550,10 @@ class TestNewCommandArchival:
         loop.sessions.save(session)
 
         archived_count = -1
-        archive_done = asyncio.Event()
 
         async def _fake_consolidate(messages) -> bool:
             nonlocal archived_count
             archived_count = len(messages)
-            archive_done.set()
             return True
 
         loop.memory_consolidator.consolidate_messages = _fake_consolidate  # type: ignore[method-assign]
@@ -561,8 +564,7 @@ class TestNewCommandArchival:
         assert response is not None
         assert "new session started" in response.content.lower()
 
-        # Wait for background archival to complete
-        await asyncio.wait_for(archive_done.wait(), timeout=1.0)
+        await loop.close_mcp()
         assert archived_count == 3
 
     @pytest.mark.asyncio
@@ -587,3 +589,31 @@ class TestNewCommandArchival:
         assert response is not None
         assert "new session started" in response.content.lower()
         assert loop.sessions.get_or_create("cli:test").messages == []
+
+    @pytest.mark.asyncio
+    async def test_close_mcp_drains_pending_archives(self, tmp_path: Path) -> None:
+        """close_mcp waits for background archive tasks to complete."""
+        from nanobot.bus.events import InboundMessage
+
+        loop = self._make_loop(tmp_path)
+        session = loop.sessions.get_or_create("cli:test")
+        for i in range(3):
+            session.add_message("user", f"msg{i}")
+            session.add_message("assistant", f"resp{i}")
+        loop.sessions.save(session)
+
+        archived = asyncio.Event()
+
+        async def _slow_consolidate(_messages) -> bool:
+            await asyncio.sleep(0.1)
+            archived.set()
+            return True
+
+        loop.memory_consolidator.consolidate_messages = _slow_consolidate  # type: ignore[method-assign]
+
+        new_msg = InboundMessage(channel="cli", sender_id="user", chat_id="test", content="/new")
+        await loop._process_message(new_msg)
+
+        assert not archived.is_set()
+        await loop.close_mcp()
+        assert archived.is_set()