Add Copilot coding agent instructions

.github/copilot-instructions.md

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+# Copilot Instructions
+
+## Repository Overview
+
+This repository provides Python bindings for the [easySBA](https://users.ics.forth.gr/~lourakis/sba/) sparse bundle adjustment C library, using [pybind11](https://pybind11.readthedocs.io/).
+
+## Repository Layout
+
+- `src/` – Original C/C++ sources for the easySBA library and the license.
+- `easysba/` – Python package: `__init__.py` re-exports `easy_sba` from the compiled pybind11 extension (`_easysba.cpp`).
+- `tests/` – `unittest`-based test suite run with `pytest`.
+- `docs/` – Usage documentation (`USAGE.md`).
+- `pyproject.toml` / `setup.py` – Build configuration (setuptools + pybind11 C extension, cibuildwheel for wheels).
+
+## Build Requirements
+
+- Python 3.11+
+- C/C++ toolchain
+- BLAS/LAPACK development libraries (e.g. `libblas-dev`, `liblapack-dev`, `liblapacke-dev`)
+- Python packages: `numpy`, `pybind11`, `setuptools`, `wheel`
+
+On Ubuntu/Debian:
+
+```bash
+sudo apt-get install -y build-essential libblas-dev liblapack-dev liblapacke-dev gfortran
+```
+
+Install the package locally (editable or regular):
+
+```bash
+pip install -v .
+# or with uv
+uv pip install -v .
+```
+
+## Running Tests
+
+```bash
+pytest
+```
+
+Tests that actually run the SBA solver require the environment variable `EASYSBA_RUN_SOLVER=1`:
+
+```bash
+EASYSBA_RUN_SOLVER=1 pytest
+```
+
+## Code Style and Conventions
+
+- Python code follows standard PEP 8 conventions.
+- C/C++ code in `src/` and `easysba/_easysba.cpp` follows the style of the existing sources.
+- Tests live in `tests/` and use `unittest.TestCase` classes, discovered and run via `pytest`.
+- Keep the public Python API minimal: `easysba.easy_sba(...)` is the single entry point.
+
+## Key Files
+
+| File | Purpose |
+|---|---|
+| `easysba/_easysba.cpp` | pybind11 binding: wraps the C `easy_sba` function |
+| `easysba/__init__.py` | Re-exports `easy_sba` for the public package API |
+| `src/sba_levmar.c` | Core Levenberg-Marquardt SBA implementation |
+| `tests/test_smoke.py` | Smoke tests (import check + optional solver run) |
+| `tests/test_sba_steps.py` | Step-by-step SBA tests |
+| `pyproject.toml` | Build system and cibuildwheel configuration |