fix: inject longTermBackground into memory prompt (#1734)

The format_memory_for_injection function only processed recentMonths and
earlierContext from the history section, silently dropping longTermBackground.

The LLM writes longTermBackground correctly and it persists to memory.json,
but it was never injected into the system prompt — making the user's
long-term background invisible to the AI.

Add the missing field handling and a regression test.

Co-authored-by: ppyt <14163465+ppyt@users.noreply.github.com>

backend/packages/harness/deerflow/agents/memory/prompt.py

@@ -246,6 +246,10 @@ def format_memory_for_injection(memory_data: dict[str, Any], max_tokens: int = 2
         if earlier.get("summary"):
             history_sections.append(f"Earlier: {earlier['summary']}")
 
+        background = history_data.get("longTermBackground", {})
+        if background.get("summary"):
+            history_sections.append(f"Background: {background['summary']}")
+
         if history_sections:
             sections.append("History:\n" + "\n".join(f"- {s}" for s in history_sections))