[FEATURE] Add support for private channels in Slack MCP integration

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

Currently, the Slack MCP integration's slack_list_channels tool only returns public channels. It would be valuable to also support private channels when the bot has been invited and has the appropriate permissions.

Proposed Solution

Desired Behavior:

• Return both public and private channels that the bot is a member of
• Respect the bot's groups:read OAuth scope to access private channels

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

MCP server integration

Use Case Example

Benefits:

• Full Slack workspace coverage
• Ability to work with sensitive/private discussions
• Better team collaboration in secure channels

Additional Context

we want to streamline our workflow in designing and planning, so we want claude code to access slack to automatically display the design details in canvas app on slack