chore: add root .gitignore for caches and virtualenvs

[negillett]

Adds a repository-root .gitignore so Python bytecode caches, pytest cache directories, local virtual environments, and common packaging outputs stay untracked during local development.

[cursor]

PR Summary

Low Risk
Low risk: only adds ignore rules for local Python caches, virtualenvs, and build outputs, with no runtime or production behavior changes.

Overview
Adds a new root .gitignore that ignores Python bytecode/caches (e.g., __pycache__, *.pyc, .pytest_cache), local virtual environments (.venv, venv, env), and common packaging/build outputs (*.egg-info, dist, build).

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