fix(gateway): a /branch child row carries the parent's system prompt

The gateway /branch copies the parent transcript byte-for-byte but created
the child row without a system prompt, so the branch's first turn rebuilt
the prompt (re-probing the workspace) and lost the warm prefix cache, and
logged the null-prompt warning on every branch. 2ec12952 fixed the CLI and
TUI branch writers; the gateway is the third. Read the parent row's prompt
and pass it to create_session.

(cherry picked from commit 4103a7c66c3b86659ee2cdbd7eef37d735aff237)

gateway/slash_commands_session.py

@@ -1039,6 +1039,14 @@ class GatewaySessionCommandsMixin:
         # ``_branched_from`` keeps the branch visible in /resume and /sessions after the parent is
         # reopened and re-ended. ALL routing columns go in at CREATE time: a crash before
         # switch_session() records the peer would otherwise leave the branch unroutable.
+        # 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)
         try:
             await self._session_db.create_session(
                 session_id=new_session_id,
@@ -1048,7 +1056,7 @@ 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)
+                display_name=current_entry.display_name, system_prompt=parent_prompt or None)
         except Exception as e:
             logger.error("Failed to create branch session: %s", e)
             return t("gateway.branch.create_failed", error=e)

tests/gateway/test_branch_routing_columns.py

@@ -153,3 +153,32 @@ class TestBranchRoutingColumns:
 
         _ = real_switch_session  # silence unused
 
+
+class TestBranchSystemPrompt:
+    @pytest.mark.asyncio
+    async def test_branched_session_carries_parent_system_prompt(self, store):
+        """The branch copies the parent transcript byte-for-byte so its first
+        turn can hit the warm prefix cache, but create_session() at branch
+        time dropped the system prompt: the child row's system_prompt column
+        was None even though the parent row held one. Mirrors the CLI/TUI fix
+        in 2ec12952 for the third branch-creation site (the gateway)."""
+        source = _make_source()
+        parent_entry = store.get_or_create_session(source)
+        store._db.update_system_prompt(parent_entry.session_id, "PARENT PROMPT")
+        store._db.append_message(parent_entry.session_id, role="user", content="hello")
+        store._db.append_message(parent_entry.session_id, role="assistant", content="world")
+
+        runner = _make_branch_runner(store)
+        await runner._handle_branch_command(_make_event("/branch"))
+
+        new_entry = store.get_or_create_session(source)
+        assert new_entry.session_id != parent_entry.session_id
+
+        row = store._db.get_session(new_entry.session_id)
+        assert row is not None
+        assert row["system_prompt"] == "PARENT PROMPT", (
+            "branched session lost the parent's system prompt — its first "
+            "turn will re-probe the workspace and forfeit the warm cache "
+            "the copied transcript buys"
+        )
+