docs: update suggestions to use "set" and "unset" environment commands

[zimeg]

Changelog

The env commands are suggested in a few places and some of these places were updated to use the preferred aliases set and unset.

Summary

This pull request updates leftover references to the old env add and env remove command names that were missed when set and unset became the primary commands in #460.

• .github/STYLE_GUIDE.md: Example usage updated from env add to env set
• cmd/env/unset.go: User-facing "no variables to remove" messages changed to "unset"
• internal/auth/auth.go: Debug message updated from var add SLACK_API_URL to env set SLACK_API_URL

Preview

N/A — text-only changes to output messages and docs.

Testing

• Run slack env unset in a project with no .env file and confirm the message says "no environment variables to unset"
• Use a custom API host and check debug output references env set SLACK_API_URL

Notes

Follow-up to #460.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.27%. Comparing base (1570564) to head (16adf49).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #525      +/-   ##
==========================================
- Coverage   71.27%   71.27%   -0.01%     
==========================================
  Files         222      222              
  Lines       18682    18682              
==========================================
- Hits        13316    13315       -1     
+ Misses       4187     4185       -2     
- Partials     1179     1182       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.