fix(oauth): include identity block in all API calls

Anthropic requires the identity prefix for OAuth tokens on every
request, but it was only included when a system prompt was present.
Calls without a system prompt (e.g. fact extraction during memory
consolidation) got 400 invalid_request_error every time, silently
breaking memory consolidation while the session trim still ran.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nanobot/providers/anthropic_oauth.py

@@ -381,6 +381,10 @@ class AnthropicOAuthProvider(LLMProvider):
                 {"type": "text", "text": get_claude_code_system_prefix()},
                 {"type": "text", "text": system, "cache_control": {"type": "ephemeral", "ttl": "1h"}},
             ]
+        else:
+            payload["system"] = [
+                {"type": "text", "text": get_claude_code_system_prefix()},
+            ]
 
         if tools:
             cached_tools = list(tools)