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

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• AI tools contributed to the development of this PR

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  • 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
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  • 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.

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[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.