fix(agent): join the merged-turn prefix in one place

The merge stamped the prefix with or without the "\n\n" separator
depending on whether the absorbed turn was empty, so a later non-empty
override on an empty-merge row came out as "RX". Store the unanswered
text as-is and join at override time with the same rule the merge uses.

agent/agent_runtime_helpers.py

@@ -604,7 +604,7 @@ def _merge_consecutive_users(messages: List[Dict]) -> Tuple[List[Dict], int]:
             # The clean-text persist override must replace only the absorbed turn, never the
             # unanswered text before it; kept across replay passes (an empty turn absorbs too).
             if prev_content:
-                prev[MERGED_TURN_PREFIX] = prev_content + "\n\n" if new_content else prev_content
+                prev[MERGED_TURN_PREFIX] = prev_content
             # Merged content invalidates the api_content sidecar; drop it so replay cannot use stale bytes.
             drop_stale_api_content(prev)
             # Pop the persist marker only when the durable row actually changed: a merge that

agent/session_persistence.py

@@ -80,11 +80,12 @@ def _override_replaces_content(msg: Dict, content: Any, override: Any) -> bool:
 
 
 def _content_with_turn_override(msg: Dict, content: Any, override: Any) -> Any:
-    """Replace only the absorbed current turn, leaving unanswered history intact."""
+    """Replace only the absorbed current turn, leaving unanswered history intact. The join matches
+    ``_merge_consecutive_users``; a row that no longer holds the prefix takes the plain override."""
     prefix = msg.get(MERGED_TURN_PREFIX)
     if isinstance(content, str) and isinstance(override, str) and isinstance(prefix, str):
-        if content.startswith(prefix):
-            return prefix + override
+        if content == prefix or content.startswith(prefix + "\n\n"):
+            return prefix + "\n\n" + override if override else prefix
     return override