feat(install): create agent CTA install script

[srtaalej]

Changelog

Create AI Agents call-to-action displays after successful CLI install.

Summary

This PR adds a "Create AI Agents in Slack" call-to-action to the post-install success message across all install scripts (macOS/Linux and Windows, prod and dev)

Test plan

• make test-install passes
• Verify output on Windows with install-windows.ps1

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 70.34%. Comparing base (922813e) to head (8a08d58).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #444      +/-   ##
==========================================
- Coverage   70.36%   70.34%   -0.02%     
==========================================
  Files         220      220              
  Lines       18506    18506              
==========================================
- Hits        13021    13018       -3     
- Misses       4308     4309       +1     
- Partials     1177     1179       +2     

☔ 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.

[zimeg]

@srtaalej Thanks for bringing these getting started improvements to review 🙏✨

I'm leaving a comment on formats to match before we update this IMHO but of course this is discussion! 🎨