Pin code sniffer to a stable release

[dereuromark]

Summary

• replace the moving dev-master code-sniffer constraint with stable ^0.6.8

Why

Development tooling should resolve from a tagged, reproducible compatibility range rather than an unbounded branch head.

Verification

• composer validate --strict

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.41%. Comparing base (31096c3) to head (2986e4f).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #268   +/-   ##
=========================================
  Coverage     92.41%   92.41%           
  Complexity     3673     3673           
=========================================
  Files           109      109           
  Lines         10421    10421           
=========================================
  Hits           9631     9631           
  Misses          790      790           

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