refactor: drop persist_session plumbing + fix broken btw mid-turn bypass (#16075)

Follow-up to PR #16053 (/btw as /background alias). Cleans up the
plumbing added exclusively for the old ephemeral /btw handler and
repairs a broken btw bypass that landed between my refactor and this
follow-up.

run_agent.py:
- Remove persist_session kwarg, instance attr, and _persist_session
  short-circuit. Only /btw ever passed persist_session=False; with
  /btw gone the default (always persist) is the only behavior anyone
  ever wanted.

gateway/run.py:
- Remove the unreachable 'if _cmd_def_inner.name == "btw"' block
  (PR #16059). Canonical name for a /btw message is 'background' after
  alias resolution — the comparison could never be true, and it called
  _handle_btw_command which no longer exists. The /background branch
  above it already dispatches /btw correctly.

tests/gateway/test_running_agent_session_toggles.py:
- Fix test_btw_dispatches_mid_run to mock _handle_background_command
  (the real dispatch target for /btw) instead of the deleted
  _handle_btw_command.

gateway/run.py

@@ -3498,17 +3498,11 @@ class GatewayRunner:
 
             # /background must bypass the running-agent guard — it starts a
             # parallel task and must never interrupt the active conversation.
+            # /btw is an alias of /background and resolves to the same canonical
+            # name, so this branch handles both commands.
             if _cmd_def_inner and _cmd_def_inner.name == "background":
                 return await self._handle_background_command(event)
 
-            # /btw must bypass the running-agent guard for the same reason
-            # as /background: it spawns a parallel ephemeral side-question
-            # task (see _handle_btw_command) that doesn't interrupt the
-            # active conversation and self-guards against concurrent /btw
-            # on the same chat.
-            if _cmd_def_inner and _cmd_def_inner.name == "btw":
-                return await self._handle_btw_command(event)
-
             # Session-level toggles that are safe to run mid-agent —
             # /yolo can unblock a pending approval prompt, /verbose cycles
             # the tool-progress display mode for the ongoing stream.

run_agent.py

@@ -892,7 +892,6 @@ class AIAgent:
         checkpoints_enabled: bool = False,
         checkpoint_max_snapshots: int = 50,
         pass_session_id: bool = False,
-        persist_session: bool = True,
     ):
         """
         Initialize the AI Agent.
@@ -964,7 +963,6 @@ class AIAgent:
         self.background_review_callback = None  # Optional sync callback for gateway delivery
         self.skip_context_files = skip_context_files
         self.pass_session_id = pass_session_id
-        self.persist_session = persist_session
         self._credential_pool = credential_pool
         self.log_prefix_chars = log_prefix_chars
         self.log_prefix = f"{log_prefix} " if log_prefix else ""
@@ -3353,10 +3351,7 @@ class AIAgent:
         """Save session state to both JSON log and SQLite on any exit path.
 
         Ensures conversations are never lost, even on errors or early returns.
-        Skipped when ``persist_session=False`` (ephemeral helper flows).
         """
-        if not self.persist_session:
-            return
         self._apply_persist_user_message_override(messages)
         self._session_messages = messages
         self._save_session_log(messages)

tests/gateway/test_running_agent_session_toggles.py

@@ -169,23 +169,22 @@ async def test_reasoning_rejected_mid_run():
 
 @pytest.mark.asyncio
 async def test_btw_dispatches_mid_run():
-    """/btw mid-run must dispatch to its handler, not hit the catch-all.
+    """/btw mid-run must dispatch to /background's handler, not hit the catch-all.
 
-    /btw spawns a parallel ephemeral side-question task that does NOT
-    interrupt the active conversation (see _handle_btw_command). It's the
-    whole point of the command — asking a side question while the main
-    turn is still working. Before the mid-turn bypass was added, /btw
-    fell through to the "Agent is running — wait or /stop first" catch-all,
-    making it useless in exactly the scenario it was designed for.
+    /btw is an alias of /background (see hermes_cli/commands.py). Typing
+    /btw mid-turn must spawn a parallel background task — that's the whole
+    point of the command. Before the mid-turn bypass was added for
+    /background, /btw fell through to the "Agent is running — wait or
+    /stop first" catch-all, making it useless in exactly the scenario it
+    was designed for. The alias and the bypass together make it work.
     """
     runner = _make_runner()
-    runner._handle_btw_command = AsyncMock(
-        return_value='💬 /btw: "what module owns titles?"\nReply will appear here shortly.'
+    runner._handle_background_command = AsyncMock(
+        return_value='🚀 Background task started: "what module owns titles?"'
     )
 
     result = await runner._handle_message(_make_event("/btw what module owns titles?"))
 
-    runner._handle_btw_command.assert_awaited_once()
+    runner._handle_background_command.assert_awaited_once()
     assert result is not None
-    assert "💬 /btw" in result
     assert "can't run mid-turn" not in result