By default `hermes slack manifest` opts the app into Slack's AI Assistant
container (assistant_view feature + assistant:write scope +
assistant_thread_* events). Slack then renders DMs as the right-hand
Assistant split-pane, where every exchange is a thread and bare slash
commands (/help, /new, ...) are not delivered as normal command events —
they only work when the bot is @mentioned. There was no way to opt out
short of hand-editing the generated JSON.
Add --no-assistant to emit a flat-DM manifest that omits those three
pieces, so DMs render as a normal chat and slash commands dispatch
inline. The regular messaging surface (Messages tab, slash commands,
Socket Mode, channel + DM scopes/events) is preserved in both modes.
Default behaviour is unchanged (assistant mode still on).
Tests: cover both manifest modes and the argparse wiring.
