Add Copilot Instructions file

[anrossi]

Description

Add an instructions file to help Copilot navigate and contribute to our codebase more effectively.

Testing

How does one test this file?

Documentation

Should the file's existence be mentioned in the README?

[guhetier]

Looks about right, funny how it focused on different bits than what it did for my run. I'll be interesting to re-run the generation prompt from time to time so that it enriches the file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.89%. Comparing base (ef8613c) to head (d1c8685).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5589      +/-   ##
==========================================
- Coverage   84.80%   83.89%   -0.92%     
==========================================
  Files          60       60              
  Lines       18647    18647              
==========================================
- Hits        15814    15644     -170     
- Misses       2833     3003     +170     

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