Refine AGENTS.md file

[juhoinkinen]

WIP

• avoid specific comments;
• better code placement;
• smaller functions & methods

Reasons for creating this PR

Link to relevant issue(s), if any

• Closes #

Description of the changes in this PR

Instructions how to test this PR

Known problems or uncertainties in this PR

Checklist

• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

Disclosure of AI Tool Usage

Please indicate AI use by choosing the most suitable TLP:AI category below and removing the irrelevant categories from the list. AI:ORANGE is the minimum level for merging.

• ⚪ AI:WHITE Written independently, no significant AI involvement.

Describe the AI tool(s) you used:

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.33%. Comparing base (317ad11) to head (19763d6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #128   +/-   ##
=======================================
  Coverage   98.33%   98.33%           
=======================================
  Files          15       15           
  Lines         902      902           
=======================================
  Hits          887      887           
  Misses         15       15           

Flag	Coverage Δ
unittests	98.33% <ø> (ø)

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