Add migration-guide for 4.0.0

[abergs]

This PR adds an AI generated migration-guide to serve as support document for those who are looking to upgrade and find themself lost in the changes.

I've used CLAUDE to generate it, and I'm also commiting the claude init file to support further use of claude.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.04%. Comparing base (6fc9bc8) to head (e64cce4).
⚠️ Report is 27 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #604   +/-   ##
=======================================
  Coverage   76.04%   76.04%           
=======================================
  Files         101      101           
  Lines        2601     2601           
  Branches      432      432           
=======================================
  Hits         1978     1978           
  Misses        513      513           
  Partials      110      110           

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