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.

model_tools.py

@@ -252,7 +252,7 @@ def get_tool_definitions(
     # Determine which tool names the caller wants
     tools_to_include: set = set()
 
-    if enabled_toolsets:
+    if enabled_toolsets is not None:
         for toolset_name in enabled_toolsets:
             if validate_toolset(toolset_name):
                 resolved = resolve_toolset(toolset_name)