[Integration] Slack Channel Monitor - consider using Socket Mode

[jpelletier1]

Some other harnesses, like Nanoclaw, use Slack's Socket Mode for creating messaging use cases (where you can message the agent in Slack and get a near real-time response).

References:

• https://docs.nanoclaw.dev/integrations/slack
• https://docs.slack.dev/apis/events-api/using-socket-mode/

We should investigate feasibility of using Slack Socket mode locally if means users can have a more reliable, faster (near real-time) experience sending messages to the OpenHands agent and getting responses in Slack.