fix: switch helper LLM to claude-sonnet-4-6 for 1M context support

Haiku 4.5 has a 200K context limit which causes SSE errors when
sessions grow large. Sonnet 4.6 supports 1M tokens at $3/$15 per
million.

src/mnemosyne/helper_llm.py

@@ -155,7 +155,7 @@ class HelperLLM:
     def __init__(
         self,
         api_key: str | None = None,
-        model: str = "claude-haiku-4-5-20251001",
+        model: str = "claude-sonnet-4-6-20260217",
         base_url: str = "https://api.anthropic.com",
     ) -> None:
         """Initialize the helper LLM client.