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 (0c37128) to head (4823636).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #444      +/-   ##
==========================================
+ Coverage   70.32%   70.34%   +0.01%     
==========================================
  Files         220      220              
  Lines       18506    18506              
==========================================
+ Hits        13015    13018       +3     
+ Misses       4313     4309       -4     
- Partials     1178     1179       +1     

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