Improve README and CONTRIBUTING guidance

[foreverest]

Summary

• clarify project purpose, requirements, and guarantees in README.md for better onboarding
• reorganize CONTRIBUTING.md into clearer contribution, review, and development instructions with checklist-style steps

Testing

• Not run (not requested)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4d9ed0e) to head (fe069d8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #18   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          193       193           
  Branches        40        38    -2     
=========================================
  Hits           193       193           

Flag	Coverage Δ
node20.x	100.00% <ø> (ø)
node22.x	100.00% <ø> (ø)
node24.x	100.00% <ø> (ø)

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