Add root .gitignore for caches and virtualenvs

[negillett]

Adds a repository-root .gitignore to ignore Python bytecode, pytest cache, common virtualenv directories, and packaging build outputs.

[cursor]

PR Summary

Low Risk
Low risk housekeeping change that only affects which generated files are ignored by git.

Overview
Adds a new root .gitignore to ignore Python bytecode and caches (e.g., __pycache__/, *.py[cod], .pytest_cache/), common virtualenv directories (.venv/, venv/, env/), and packaging/build outputs (*.egg-info/, .eggs/, dist/, build/).

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