FIX: Auto-enable fixed skull-strip seed when random seed is set

[mnoergaard]

This PR addresses issue #315, by automatically enabling --skull-strip-fixed-seed whenever --random-seed is explicitly provided, so skull stripping no longer remains an independent source of run-to-run randomness.

This also updates the CLI help text, adds a short usage note about reproducible runs, and includes a parser regression test.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.77%. Comparing base (f25c4e8) to head (a0b1f7e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #319      +/-   ##
==========================================
+ Coverage   84.76%   84.77%   +0.01%     
==========================================
  Files          87       87              
  Lines        8964     8974      +10     
  Branches      961      961              
==========================================
+ Hits         7598     7608      +10     
  Misses       1140     1140              
  Partials      226      226              

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