Add support for Workflow Builder form submissions

Feature Request

The Slack MCP server currently has no way to interact with Workflow Builder forms. Some channels require form submissions (via Workflow Builder) before a message can be posted, which means AI agents cannot post to those channels.

Use Case

Many Slack workspaces use Workflow Builder to enforce structured input on certain channels — e.g. standup updates, incident reports, change requests, or announcement templates. Without form submission support, AI agents are blocked from contributing to these channels even when they have the relevant content.

Proposed Tools

• slack_list_workflows — List available workflows in a channel
  • Parameters: channel_id
• slack_get_workflow_form — Get the form schema/fields for a workflow
  • Parameters: workflow_id
• slack_submit_workflow_form — Submit a completed form to trigger a workflow
  • Parameters: workflow_id, channel_id, form_data (key-value pairs matching the form fields)

Start by reviewing the existing Slack MCP server tool implementations and how current operations expose parameters and responses. Define the listing, form-schema, and submission behavior described in the issue, then verify that each tool can retrieve workflow data and submit matching form fields to the requested channel.