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.

cli.py

@@ -3904,6 +3904,8 @@ class HermesCLI:
             self._handle_stop_command()
         elif canonical == "background":
             self._handle_background_command(cmd_original)
+        elif canonical == "btw":
+            self._handle_btw_command(cmd_original)
         elif canonical == "queue":
             # Extract prompt after "/queue " or "/q "
             parts = cmd_original.split(None, 1)
@@ -4190,6 +4192,121 @@ class HermesCLI:
         self._background_tasks[task_id] = thread
         thread.start()
 
+    def _handle_btw_command(self, cmd: str):
+        """Handle /btw <question> — ephemeral side question using session context.
+
+        Snapshots the current conversation history, spawns a no-tools agent in
+        a background thread, and prints the answer without persisting anything
+        to the main session.
+        """
+        parts = cmd.strip().split(maxsplit=1)
+        if len(parts) < 2 or not parts[1].strip():
+            _cprint("  Usage: /btw <question>")
+            _cprint("  Example: /btw what module owns session title sanitization?")
+            _cprint("  Answers using session context. No tools, not persisted.")
+            return
+
+        question = parts[1].strip()
+        task_id = f"btw_{datetime.now().strftime('%H%M%S')}_{uuid.uuid4().hex[:6]}"
+
+        if not self._ensure_runtime_credentials():
+            _cprint("  (>_<) Cannot start /btw: no valid credentials.")
+            return
+
+        turn_route = self._resolve_turn_agent_config(question)
+        history_snapshot = list(self.conversation_history)
+
+        preview = question[:60] + ("..." if len(question) > 60 else "")
+        _cprint(f'  💬 /btw: "{preview}"')
+
+        def run_btw():
+            try:
+                btw_agent = AIAgent(
+                    model=turn_route["model"],
+                    api_key=turn_route["runtime"].get("api_key"),
+                    base_url=turn_route["runtime"].get("base_url"),
+                    provider=turn_route["runtime"].get("provider"),
+                    api_mode=turn_route["runtime"].get("api_mode"),
+                    acp_command=turn_route["runtime"].get("command"),
+                    acp_args=turn_route["runtime"].get("args"),
+                    max_iterations=8,
+                    enabled_toolsets=[],
+                    quiet_mode=True,
+                    verbose_logging=False,
+                    session_id=task_id,
+                    platform="cli",
+                    reasoning_config=self.reasoning_config,
+                    providers_allowed=self._providers_only,
+                    providers_ignored=self._providers_ignore,
+                    providers_order=self._providers_order,
+                    provider_sort=self._provider_sort,
+                    provider_require_parameters=self._provider_require_params,
+                    provider_data_collection=self._provider_data_collection,
+                    fallback_model=self._fallback_model,
+                    session_db=None,
+                    skip_memory=True,
+                    skip_context_files=True,
+                    persist_session=False,
+                )
+
+                btw_prompt = (
+                    "[Ephemeral /btw side question. Answer using the conversation "
+                    "context. No tools available. Be direct and concise.]\n\n"
+                    + question
+                )
+                result = btw_agent.run_conversation(
+                    user_message=btw_prompt,
+                    conversation_history=history_snapshot,
+                    task_id=task_id,
+                    sync_honcho=False,
+                )
+
+                response = (result.get("final_response") or "") if result else ""
+                if not response and result and result.get("error"):
+                    response = f"Error: {result['error']}"
+
+                # TUI refresh before printing
+                if self._app:
+                    self._app.invalidate()
+                    time.sleep(0.05)
+                print()
+
+                if response:
+                    try:
+                        from hermes_cli.skin_engine import get_active_skin
+                        _skin = get_active_skin()
+                        _resp_color = _skin.get_color("response_border", "#4F6D4A")
+                    except Exception:
+                        _resp_color = "#4F6D4A"
+
+                    ChatConsole().print(Panel(
+                        _rich_text_from_ansi(response),
+                        title=f"[{_resp_color} bold]⚕ /btw[/]",
+                        title_align="left",
+                        border_style=_resp_color,
+                        box=rich_box.HORIZONTALS,
+                        padding=(1, 2),
+                    ))
+                else:
+                    _cprint("  💬 /btw: (no response)")
+
+                if self.bell_on_complete:
+                    sys.stdout.write("\a")
+                    sys.stdout.flush()
+
+            except Exception as e:
+                if self._app:
+                    self._app.invalidate()
+                    time.sleep(0.05)
+                print()
+                _cprint(f"  ❌ /btw failed: {e}")
+            finally:
+                if self._app:
+                    self._invalidate(min_interval=0)
+
+        thread = threading.Thread(target=run_btw, daemon=True, name=f"btw-{task_id}")
+        thread.start()
+
     @staticmethod
     def _try_launch_chrome_debug(port: int, system: str) -> bool:
         """Try to launch Chrome/Chromium with remote debugging enabled.