fix: use correct role for summary message in context compressor

The summary message was always injected as 'user' role, which causes
consecutive user messages when the last preserved head message is also
'user'. Some APIs reject this (400 error), and it produces malformed
training data.

Fix: check the role of the last head message and pick the opposite role
for the summary — 'user' after assistant/tool, 'assistant' after user.

Based on PR #328 by johnh4098. Closes #328.

agent/context_compressor.py

@@ -342,7 +342,9 @@ Write only the summary, starting with "[CONTEXT SUMMARY]:" prefix."""
             compressed.append(msg)
 
         if summary:
-            compressed.append({"role": "user", "content": summary})
+            last_head_role = messages[compress_start - 1].get("role", "user") if compress_start > 0 else "user"
+            summary_role = "user" if last_head_role in ("assistant", "tool") else "assistant"
+            compressed.append({"role": summary_role, "content": summary})
         else:
             if not self.quiet_mode:
                 print("   ⚠️  No summary model available — middle turns dropped without summary")