Add AGENTS.md and GitHub Copilot instructions

.github/copilot-instructions.md

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+# GitHub Copilot Instructions — mkl_fft
+
+## Identity
+You are an expert Python/C developer working on `mkl_fft` at Intel.
+Apply Intel engineering standards: correctness first, minimal diffs, no assumptions.
+
+## Source of truth
+This file is canonical for Copilot/agent behavior.
+`AGENTS.md` provides project context.
+
+## Precedence
+copilot-instructions > nearest AGENTS > root AGENTS
+Higher-precedence file overrides; lower must not restate overridden guidance.
+
+## Mandatory flow
+1. Read root `AGENTS.md`. If absent, stop and report.
+2. For edited files, use root AGENTS (no local AGENTS files exist here).
+3. If future local `AGENTS.md` files appear, find nearest per file.
+
+## Contribution expectations
+- Keep diffs minimal; prefer atomic single-purpose commits.
+- Preserve NumPy/SciPy FFT API compatibility by default.
+- For API changes: update tests + docstrings when user-visible.
+- For bug fixes: add regression tests in `mkl_fft/tests/`.
+- Do not generate code without a corresponding test update in the same step.
+- Run `pre-commit run --all-files` before proposing code changes.
+
+## Authoring rules
+- Use source-of-truth files for all mutable details.
+- Never invent/hardcode versions, flags, or matrix values.
+- Use stable entry points: `pip install -e .` (dev), `pytest mkl_fft/tests` (test).
+- Never include sensitive data in any file.
+- **C templates:** Edit only `src/*.c.src`; do not manually edit generated `.c` files.
+- **Cython/MKL calls:** Release GIL with `with nogil:` blocks for performance-critical MKL functions.
+- **Memory:** Ensure proper alignment for FFT buffers; respect MKL object lifecycle.
+- **Compiler flags:** Do not hardcode ISA flags (AVX-512, etc.) outside `setup.py`.
+
+## Source-of-truth files
+- Build/config: `pyproject.toml`, `setup.py`
+- Dependencies: `pyproject.toml` (dependencies, optional-dependencies), `conda-recipe/meta.yaml`, `conda-recipe-cf/meta.yaml`
+- CI: `.github/workflows/*.{yml,yaml}`
+- Style/lint: `.pre-commit-config.yaml`, `.flake8`, `pyproject.toml` (tool.black, tool.isort, tool.cython-lint)
+- API contracts: `mkl_fft/__init__.py`, `mkl_fft/interfaces/*.py`, NumPy FFT docs (https://numpy.org/doc/stable/reference/routines.fft.html), SciPy FFT docs (https://docs.scipy.org/doc/scipy/reference/fft.html)
+- Test data: `mkl_fft/tests/`
+
+## Intel-specific constraints
+- Package channels: Intel PyPI (https://software.repos.intel.com/python/pypi), Intel conda (https://software.repos.intel.com/python/conda), conda-forge
+- MKL backend: requires `mkl-devel` at build time, `mkl-service` at runtime
+- Performance: for claims, reference https://github.com/intelpython/fft_benchmark
+- Do not hardcode MKL version assumptions; respect `pyproject.toml` `requires-python` range

AGENTS.md

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+# AGENTS.md — mkl_fft
+
+## What this project is
+NumPy-based Python interface to Intel® oneMKL FFT functions. Provides MKL-accelerated FFT for real/complex transforms. Part of Intel® Distribution for Python. Archetype: **python** (Cython + C extensions).
+
+Layers: Python interfaces, Cython bindings (`_pydfti.pyx`), C backend (`src/*.c.src`).
+
+## How it's structured
+- `mkl_fft/interfaces/` — drop-in replacements for `numpy.fft`, `scipy.fft`
+- `mkl_fft/src/` — C templates (`.c.src`)
+- `mkl_fft/tests/` — pytest suite
+- `conda-recipe/`, `conda-recipe-cf/` — Intel/conda-forge builds
+
+Build: `pyproject.toml` + `setup.py`. Runtime: `mkl-service`, `numpy>=1.26.4`.
+
+## How to work in it
+- Keep changes atomic and single-purpose.
+- Preserve NumPy/SciPy FFT API; document divergence in commit message.
+- Pair changes with tests and docstrings.
+- Never assume MKL or NumPy versions; use source-of-truth files.
+- **C templates:** Edit only `*.c.src` files; generated `.c` is ephemeral (via `_vendored/conv_template.py`).
+- **Local dev:** `conda create -n dev python numpy cython mkl-devel pytest && pip install -e .`
+
+For agent policy: `.github/copilot-instructions.md`
+
+## Where truth lives
+- Build/config: `pyproject.toml`, `setup.py`
+- Dependencies: `pyproject.toml` (`dependencies`, `optional-dependencies`), `conda-recipe/meta.yaml`, `conda-recipe-cf/meta.yaml`
+- CI: `.github/workflows/`
+- Style/lint: `.pre-commit-config.yaml`, `.flake8`, `pyproject.toml` (black/isort/cython-lint)
+- API/contracts: `mkl_fft/__init__.py`, `mkl_fft/interfaces/`, NumPy/SciPy FFT docs
+- Stable entry points: `python -m pip install .`, `pytest mkl_fft/tests`
+
+## Directory map
+No local AGENTS files — project is small enough for root-level guidance only.