feat: Plugin for posting updates to a Slack channel [SC-34358]

[mjp]

This plugin uses the Slack "incoming webhook" functionality to post updates to Slack channels. A new MCP tool is defined for each Slack channel configured.

plugins:
  slack:
    enabled: true
    settings:
      channels:
        - name: alerts
          description: Send an update to the alerts Slack channel
          webhook_url: https://hooks.slack.com/services/...

$ unpage mcp tools call slack_post_to_alerts --arguments '["test message", "Unpage"]'
Successfully posted message to #alerts

[tvdaptible]

woo thank you! i'll make a separate pr that adds docs and interactive config.