Add new slack tools that allow posting messages on behalf of user

[zee-fee]

@luosc Modified README to include steps for generating user token and adding relevant scopes

Description

Slack MCP tool does not support the tool to post messages to channels/private messages on behalf of the user. This requires a new User Token to be added and new tools to be implemented.

Server Details

• Server: slack
• Changes to: tools and documenation

Motivation and Context

It would be a nice feature to allow the tool to post messages on behalf of the user. Currently, only the bot can post messages. Posting messages on behalf of the user makes it more personal and adds a human touch which messages from bot currently don't have.

How Has This Been Tested?

I have tested this with Claude Desktop client and local ollama mcp-client

Breaking Changes

No. The user token is optional

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options

Additional context

[zee-fee]

Please hold merging for now. I am still implementing the feature

[olaservo]

Thanks for contributing this PR. This server has been moved to the archived repository at https://github.com/modelcontextprotocol/servers-archived to reduce maintenance overhead, so we can focus our efforts on a smaller set of core servers.
Since we're no longer accepting contributions to this server, we won't be able to merge this PR. If you're interested in contributing similar functionality, we recommend checking out the alternatives listed in the Readme.