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. Integration tests are failing because they ran on my own fork, and for security sake, I've not added the required secrets for integration tests to pass.

[aron-muon]

Closed to change fork base to a branch on my fork rather than master