fix(compressor): summary role can create consecutive same-role messages

The summary message role was determined only by the last head message,
ignoring the first tail message. This could create consecutive user
messages (rejected by Anthropic) when the tail started with 'user'.

Now checks both neighbors. Priority: avoid colliding with the head
(already committed). If the chosen role also collides with the tail,
flip it — but only if flipping wouldn't re-collide with the head.

agent/context_compressor.py

@@ -313,7 +313,19 @@ Write only the summary body. Do not include any preamble or prefix; the system w
 
         if 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"
+            first_tail_role = messages[compress_end].get("role", "user") if compress_end < n_messages else "user"
+            # Pick a role that avoids consecutive same-role with both neighbors.
+            # Priority: avoid colliding with head (already committed), then tail.
+            if last_head_role in ("assistant", "tool"):
+                summary_role = "user"
+            else:
+                summary_role = "assistant"
+            # If the chosen role collides with the tail AND flipping wouldn't
+            # collide with the head, flip it.
+            if summary_role == first_tail_role:
+                flipped = "assistant" if summary_role == "user" else "user"
+                if flipped != last_head_role:
+                    summary_role = flipped
             compressed.append({"role": summary_role, "content": summary})
         else:
             if not self.quiet_mode: