refactor(gateway): read the /branch parent prompt inline

The try/except around the parent get_session() only added a silent path
that reproduces the bug (a child row with no system prompt). Read it inline
like the handler's other DB calls, per AGENTS.md no-defensive-wrappers.

gateway/slash_commands_session.py

@@ -1042,11 +1042,7 @@ class GatewaySessionCommandsMixin:
         # The child sends the parent's exact system prompt: a row without one makes the branch's
         # first turn rebuild (re-probing the workspace) and forfeits the warm cache the copied
         # transcript buys.
-        parent_prompt = None
-        try:
-            parent_prompt = ((await self._session_db.get_session(parent_session_id)) or {}).get("system_prompt")
-        except Exception:
-            logger.debug("branch: parent system prompt read failed for %s", parent_session_id, exc_info=True)
+        parent = await self._session_db.get_session(parent_session_id)
         try:
             await self._session_db.create_session(
                 session_id=new_session_id,
@@ -1056,7 +1052,7 @@ class GatewaySessionCommandsMixin:
                 parent_session_id=parent_session_id, user_id=dest_source.user_id,
                 session_key=dest_key, chat_id=dest_source.chat_id, chat_type=dest_source.chat_type,
                 thread_id=dest_source.thread_id, origin_json=_branch_origin_json,
-                display_name=current_entry.display_name, system_prompt=parent_prompt or None)
+                display_name=current_entry.display_name, system_prompt=(parent or {}).get("system_prompt") or None)
         except Exception as e:
             logger.error("Failed to create branch session: %s", e)
             return t("gateway.branch.create_failed", error=e)