[FEATURE] Slack plugin: add slack_trigger_workflow tool

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Many Slack channels use Workflow Builder workflows (triggered via channel shortcuts or webhook URLs) to post structured messages with consistent formatting, auto-pinning, and form metadata. The @anthropic/claude-code-slack plugin can send plain messages but cannot trigger these workflows, forcing agents to either post manually-formatted messages that lack workflow behavior (auto-pin, form tracking) or ask the user to trigger the workflow themselves.

Proposed Solution

Add a tool:

slack_trigger_workflow — triggers a Slack workflow via its webhook URL

• Parameters: webhook_url (string), inputs (object, key-value pairs matching the workflow's input variables)
• Maps to Slack's Workflow webhook triggers — a simple POST to the webhook URL with a JSON body containing the input values

Example use case: a team has a "PR Review Request" workflow that takes description and pr_link inputs, posts a formatted message, and auto-pins it. The agent could trigger it directly instead of mimicking the format manually.

Alternative Solutions

1. Agent posts a manual slack_send_message mimicking the workflow format + user manually pins
2. User triggers the workflow themselves after the agent provides the form values

Priority

Low - Nice to have

Feature Category

Plugins / MCP Tools