Add .github/copilot-instructions.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: aclark4life

.github/copilot-instructions.md

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+# Copilot Instructions for Pillow
+
+## What this project is
+
+Pillow is the Python Imaging Library fork — a Python 3.10+ library for opening,
+manipulating, and saving many image file formats. It is a mix of Python and C:
+pure Python format plugins live in `src/PIL/`, and eight C extension modules
+(`_imaging`, `_imagingcms`, `_imagingft`, `_imagingmath`, `_imagingmorph`,
+`_imagingtk`, `_avif`, `_webp`) are compiled at install time via `setup.py`.
+
+## Project layout
+
+```
+src/PIL/              Python source and C extension stubs (.pyi)
+src/thirdparty/       Vendored C libraries (raqm, fribidi-shim, pythoncapi_compat)
+Tests/                pytest test suite; Tests/helper.py has shared utilities
+docs/                 Sphinx documentation (RST)
+docs/releasenotes/    Per-release changelog entries
+setup.py              C extension build configuration
+_custom_build/        Custom setuptools build backend
+src/PIL/_version.py   Version number (PEP 440)
+pyproject.toml        Project metadata and optional dependency groups
+.pre-commit-config.yaml  All linting and formatting hooks
+```
+
+## Install
+
+```bash
+python3 -m pip install -e ".[tests]"
+```
+
+The C extensions are compiled during install. Native libraries (libjpeg,
+libpng, libtiff, libwebp, freetype2, littlecms2, etc.) must already be present
+on the system.
+
+## Test
+
+```bash
+python3 selftest.py          # quick sanity check
+python3 -m pytest Tests/     # full test suite
+python3 -m pytest Tests/ -n auto  # parallel (requires pytest-xdist)
+```
+
+Always run `python3 selftest.py` first. New features and bug fixes must include
+tests. Test files follow the naming pattern `Tests/test_*.py`.
+
+## Lint and format
+
+```bash
+make lint        # runs tox -e lint → pre-commit (black, ruff, clang-format,
+                 #   bandit, sphinx-lint, pyproject-fmt, and more)
+make lint-fix    # auto-fixes black and ruff violations
+```
+
+Or run pre-commit directly:
+
+```bash
+pre-commit run --all-files
+```
+
+All hooks must pass before a PR can be merged. Separate reformatting commits
+from functional commits.
+
+## Type checking
+
+```bash
+python3 -m tox -e mypy
+```
+
+## Documentation
+
+Docs use Sphinx/RST. Build locally with `make docserve`. Release notes go in
+`docs/releasenotes/<version>.rst` using the existing section structure
+(Security, Backwards incompatible changes, Deprecations, API changes, API
+additions, Other changes). Use the `:cve:` RST role for CVE references.
+Include a release note entry for any user-visible change.
+
+## CI checks (must all pass before merge)
+
+GitHub Actions runs: `test.yml` (pytest on Linux/macOS/Windows), `lint.yml`
+(pre-commit), `docs.yml` (Sphinx build), and `wheels.yml` on tag push.
+
+---
+
+## Pillow-specific guidance
+
+Use `python3` and `python3 -m pip install` — never `python` or bare `pip`.
+
+`Image.open()` takes a `formats=` argument (list/tuple of format IDs to
+allowlist). There is no `format=` parameter.
+
+`Image.OPEN` is an internal format registry. Do not recommend that users
+mutate it as a security or configuration mechanism.
+
+`pybind11` is a build-time-only dependency used for parallel C compilation,
+not for C++/Python bindings.
+
+Use fully qualified exception names: `Image.DecompressionBombError` and
+`Image.DecompressionBombWarning`, not the bare class names.
+
+Do not embed specific numeric values for thresholds like `MAX_IMAGE_PIXELS` —
+they change across releases. Reference the named constant instead.