fix(compression): always append END OF CONTEXT SUMMARY marker to standalone summaries regardless of role

When the compression summary lands as an assistant-role message (head ends
with user), the end marker was not appended. Models may regurgitate the
summary text as their own visible output when there's no clear boundary
signal (#33256).

The end marker was already appended for user-role summaries (#11475, #14521)
but the assistant-role path was missed in the original fix. This ensures ALL
standalone summary messages carry the boundary marker, preventing summary
text from leaking into user-visible chat output.

agent/context_compressor.py

@@ -2193,10 +2193,12 @@ This compaction should PRIORITISE preserving all information related to the focu
 
         # When the summary lands as a standalone role="user" message,
         # weak models read the verbatim "## Active Task" quote of a past
-        # user request as fresh input (#11475, #14521). Append the explicit
-        # end marker — the same one used in the merge-into-tail path — so
-        # the model has a clear "summary above, not new input" signal.
-        if not _merge_summary_into_tail and summary_role == "user":
+        # user request as fresh input (#11475, #14521).
+        # When it lands as role="assistant", models may regurgitate the
+        # summary text as their own output (#33256). In both cases, append
+        # the explicit end marker so the model has a clear "summary ends
+        # here, respond to the message below" signal.
+        if not _merge_summary_into_tail:
             summary = (
                 summary
                 + "\n\n--- END OF CONTEXT SUMMARY — "