hermes - 💡(How to fix) Fix Gateway should allow suppressing background review notifications [1 pull requests]

Problem

Gateway background review notifications currently use agent.background_review_callback to send post-turn messages such as memory or skill update summaries back into the same chat after the main response. That is useful in some personal contexts, but it is noisy in shared chat surfaces where users often only want the final answer.

There is no simple display setting to suppress only these extra post-turn notification messages while leaving background review itself enabled.

Related context

• #18073 surfaced self-improvement review summaries across CLI, TUI, and gateway.
• #4684 proposes configurable background memory update notifications, but appears broader and memory-focused.
• #20533 / #20559 discuss chat-type display overrides, which is related but larger than this request.

Proposed behaviour

Add a gateway display setting:

display:
  background_review_notifications: true

And allow existing per-platform display override shape:

display:
  platforms:
    slack:
      background_review_notifications: false

When the setting resolves to false, Hermes should suppress the user-facing background review notification delivery by not attaching agent.background_review_callback for that turn. Background review itself should still run normally; only the extra gateway message is suppressed.

Non-goals

• No per-channel or per-chat-id config schema in this issue.
• No change to memory/skill review behaviour itself.
• No Slack-specific sentinel handling.
• No broad display config redesign.

Suggested implementation

• Add background_review_notifications to gateway.display_config defaults and boolean normalisation.
• In gateway/run.py, resolve the setting using the existing resolve_display_setting() path.
• Assign agent.background_review_callback = None when disabled.
• Skip registering the post-delivery release callback when disabled.
• Document the setting in cli-config.yaml.example.

Verification

• Resolver tests for default, global false, true/false string normalisation, and per-platform override.
• Gateway runtime tests proving the callback is registered when enabled and not registered when disabled.