feat: govslack compatibility

[aron-muon]

Summary

Adds support for GovSlack (Slack's FedRAMP-compliant government offering) by making API domains configurable via environment variable.

Closes #161

Changes

pkg/provider/edge/edge.go

• Added getSlackBaseDomain() helper function that returns the appropriate base domain
• Updated NewWithClient() and NewWithInfo() to use dynamic domains for Edge API endpoints

pkg/provider/api.go

• Modified NewMCPSlackClient() to use GovSlack API URL for initial authentication when enabled

Usage

Set the environment variable to enable GovSlack mode:

export SLACK_MCP_GOVSLACK=true

This configures the server to use GovSlack API endpoints:

Component	Commercial Slack	GovSlack
Standard API	slack.com/api/	slack-gov.com/api/
Edge API	edgeapi.slack.com	edgeapi.slack-gov.com
Workspace URL	{team}.slack.com	{team}.slack-gov.com

Test Plan

• Code compiles successfully
• Tested with GovSlack workspace using xoxp token
• Tested with GovSlack workspace using xoxb token
• Verified Edge API calls work correctly

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🤖 Generated with Claude Code

[aron-muon]

I've verified that the functionality works properly in GovSlack. I was able to query for channels, read messages, send messages, etc.

[korotovsky]

Hi @aron-muon, this is a super cool PR! Thank you!