Add pixi environment directory to .gitignore

[bernalde]

Fixes # .

None — minor development-environment housekeeping; no linked issue.

Summary/Motivation:

Pixi is a conda-ecosystem environment/package manager that stores its generated environments in a .pixi/ directory at the project root. Like the IDE configuration directories already ignored (.vscode, .idea, .spyder*), this directory is machine-generated, potentially large, and specific to a contributor's local setup, so checkouts of contributors who manage their Pyomo development environment with pixi currently show thousands of untracked files.

Changes proposed in this PR:

• Add .pixi/ to .gitignore, alongside the existing editor/tool entries.

AI-Use Disclosure

• AI tools were NOT used during the preparation of this PR

or

• AI tools contributed to the development of this PR

  • AI tools generated documentation (including the PR description/comments, code comments, and/or Sphinx documentation)
  • AI tools generated tests (baselines, examples, and/or code)
  • AI tools generated code (apart from tests)

  Review process (select ONE):

  • Rewritten: All AI-generated content was rewritten by me before being committed.
  • Reviewed/verified: I retained AI-generated content and verified it before committing. Verification included (as applicable):
    • Ran the code and fixed issues
    • Added and ran tests
    • Checked correctness/logic of code and tests
    • Checked for alignment with the contribution guide
    • Considered security implications
  • As-is: AI-generated content was commited directly to the repository

Notes for reviewers (optional): AI assistance was limited to drafting this description and the commit message; the one-line .gitignore entry itself was authored manually in a local checkout before being upstreamed here.

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[bernalde]

Addressed the review in 84047bb: .pixi/ now sits in the IDE configuration block (after .vscode), and the separate commented section at the bottom is gone — the PR's net diff against main is the single added line. Verified locally with git check-ignore -v, which resolves .pixi/envs/... to the new entry.

No tests were added; the change is a .gitignore entry with no testable behavior.

CI for the new head was starting when this was posted. The formal review decision remains CHANGES_REQUESTED until @mrmundt re-reviews; review re-requested.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.94%. Comparing base (2744a24) to head (84047bb).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4025   +/-   ##
=======================================
  Coverage   89.94%   89.94%           
=======================================
  Files         917      917           
  Lines      109250   109250           
=======================================
+ Hits        98263    98264    +1     
+ Misses      10987    10986    -1     

Flag	Coverage Δ
builders	29.12% <ø> (+<0.01%)	⬆️
default	86.14% <ø> (?)
expensive	35.10% <ø> (?)
linux	87.67% <ø> (-1.98%)	⬇️
linux_other	87.67% <ø> (ø)
oldsolvers	27.58% <ø> (+<0.01%)	⬆️
osx	83.12% <ø> (ø)
win	85.43% <ø> (ø)

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