feat: add user group management tools

[RedSlowpoke]

Summary

Add five new tools for managing Slack user groups (subteams):

• usergroups_list - List all user groups in the workspace with optional filters
• usergroups_me - Manage your own membership: list groups you're in, join, or leave
• usergroups_create - Create a new user group with name, handle, and description
• usergroups_update - Update group metadata (name, handle, description, default channels)
• usergroups_users_update - Replace all members of a group

Key Features

The usergroups_me tool provides a convenient way to join or leave groups without needing to know the full member list. It handles fetching current members and updating the list automatically - addressing the limitation that Slack's API only supports replacing the entire member list.

Required OAuth Scopes

• usergroups:read - For list operations
• usergroups:write - For create/update/join/leave operations

Tool Descriptions

All tools include detailed descriptions optimized for LLM understanding, explaining:

• What user groups are (mention groups like @engineering)
• When to use each tool
• Output format (CSV vs JSON)
• ID format and how to get IDs
• Warnings about destructive operations

Test Plan

• Build passes: go build ./...
• Vet passes: go vet ./...
• Manual testing with MCP Inspector
• Verified idempotent join/leave (returns message if already member/not member)

[korotovsky]

Bringing new tools into the MCP is one of the most complicated things, because I try to keep them functional on the same level among all three types of tokens: xoxp, xoxb and xoxd/xoxc. Have you researched possible solutions how usergroups could be managed via xoxc/xoxd?

[RedSlowpoke]

I've manually tested with xoxc/xoxd and it works just fine.

[RedSlowpoke]

Additionaly, I want to add, that some of the tools that I propose in this PR are desctructive.
I think it may be the best to finalize the #173 before finalizing this one.

[korotovsky]

I agree, was about to suggest the same. But please update OAuth JSON spec in docs for completeness anyway.

[RedSlowpoke]

I've tested with xoxb as well and it make it possible for list and even _me works to join and leave for the bot.
So I've tested with all 3 types of tokens manually and it seems working for me on my corporate slack.