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.

hermes_cli/commands.py

@@ -67,6 +67,8 @@ COMMAND_REGISTRY: list[CommandDef] = [
                gateway_only=True),
     CommandDef("background", "Run a prompt in the background", "Session",
                aliases=("bg",), args_hint="<prompt>"),
+    CommandDef("btw", "Ephemeral side question using session context (no tools, not persisted)", "Session",
+               args_hint="<question>"),
     CommandDef("queue", "Queue a prompt for the next turn (doesn't interrupt)", "Session",
                aliases=("q",), args_hint="<prompt>"),
     CommandDef("status", "Show session info", "Session",