feat: add /btw command for ephemeral side questions (#4161)

Adds /btw <question> — ask a quick follow-up using the current
session context without interrupting the main conversation.

- Snapshots conversation history, answers with a no-tools agent
- Response is not persisted to session history or DB
- Runs in a background thread (CLI) / async task (gateway)
- Per-session guard prevents concurrent /btw in gateway

Implementation:
- model_tools.py: enabled_toolsets=[] now correctly means "no tools"
  (was falsy, fell through to default "all tools")
- run_agent.py: persist_session=False gates _persist_session()
- cli.py: _handle_btw_command (background thread, Rich panel output)
- gateway/run.py: _handle_btw_command + _run_btw_task (async task)
- hermes_cli/commands.py: CommandDef for "btw"

Inspired by PR #3504 by areu01or00, reimplemented cleanly on current
main with the enabled_toolsets=[] fix and without the __btw_no_tools__
hack.

run_agent.py

@@ -508,6 +508,7 @@ class AIAgent:
         checkpoints_enabled: bool = False,
         checkpoint_max_snapshots: int = 50,
         pass_session_id: bool = False,
+        persist_session: bool = True,
     ):
         """
         Initialize the AI Agent.
@@ -573,6 +574,7 @@ 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.log_prefix_chars = log_prefix_chars
         self.log_prefix = f"{log_prefix} " if log_prefix else ""
         # Store effective base URL for feature detection (prompt caching, reasoning, etc.)
@@ -1700,7 +1702,10 @@ 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)