Tweaks to simplify working with coding agents

[speth]

Changes proposed in this pull request

• Provide workaround for calling sysctl, which is not allowed within Codex's sandbox
• Avoid repeated instantiations of std::locale objects which slowed down running tests within Codex's sandbox
• Add a couple of configuration modifications to make working with Pixi easier. These are just the changes that pixi init wants to make to already-tracked files. I'm holding off on adding pixi.toml or pixi.lock files for the time being. The motivation for this is in part that I think it's a little easier to get agents to run build and test commands when they don't have to figure out what Conda environment they're supposed to activate.
• A couple of other minor changes that I didn't want to roll into Convert kinetics & equilibrium code to use std::span #2085.

AI Statement (required)

• No generative AI was used. This contribution was written entirely without AI assistance.

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• AI Statement is included
• The pull request is ready for review

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.43%. Comparing base (9470b53) to head (46e6bc1).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2086   +/-   ##
=======================================
  Coverage   77.42%   77.43%           
=======================================
  Files         451      451           
  Lines       52718    52714    -4     
  Branches     8839     8838    -1     
=======================================
+ Hits        40818    40819    +1     
+ Misses       8931     8926    -5     
  Partials     2969     2969           

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[ischoegl]

Thanks - this all looks good to me!