Update code reviewer agent

[eleftherias]

• Remove positive observations, to make review comments shorter
• Add section about backwards compatibility
• Add section about linking PRs to issues

[eleftherias]

@JAORMX @jhrozek I updated the agent with some of my own preferences, I don't feel too strongly about them, let me know if you disagree

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.54%. Comparing base (2118268) to head (319bd77).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3585      +/-   ##
==========================================
+ Coverage   65.48%   65.54%   +0.05%     
==========================================
  Files         404      404              
  Lines       39873    39873              
==========================================
+ Hits        26111    26133      +22     
+ Misses      11742    11719      -23     
- Partials     2020     2021       +1     

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