fix(memory): fall back to raw_archive on LLM error response

When chat_with_retry returns an error response (finish_reason='error')
instead of raising an exception, archive() previously treated the error
message as a valid summary and wrote it to history.jsonl, while the
original session data was already cleared by /new — causing irreversible
data loss.

Fix: check finish_reason after the LLM call and raise RuntimeError on
error responses, which naturally falls through to the existing raw_archive
fallback. This preserves the original messages in history.jsonl instead
of losing them.

Fixes #3244

nanobot/agent/memory.py

@@ -457,6 +457,8 @@ class Consolidator:
                 tools=None,
                 tool_choice=None,
             )
+            if response.finish_reason == "error":
+                raise RuntimeError(f"LLM returned error: {response.content}")
             summary = response.content or "[no summary]"
             self.store.append_history(summary)
             return summary