Add AGENTS.md with Cursor Cloud specific instructions

Co-authored-by: AdaWorldAPI <AdaWorldAPI@users.noreply.github.com>

Author: cursoragent

AGENTS.md

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+# AGENTS.md
+
+## Cursor Cloud specific instructions
+
+This is a Rust library crate (ndarray fork with HPC extensions). No external services (databases, APIs) are needed.
+
+### Quick reference
+
+| Action | Command |
+|--------|---------|
+| Build | `cargo build` |
+| Lint | `cargo clippy -- -D warnings` |
+| Test (lib) | `cargo test --lib -p ndarray` |
+| Test (workspace) | `cargo test` |
+| Test (HPC subset) | `cargo test --lib -p ndarray -- hpc::` |
+| Run example | `cargo run --example life` |
+| Format check | `cargo fmt -- --check` |
+
+### Environment notes
+
+- **Rust 1.94.0** is pinned via `rust-toolchain.toml`; rustup auto-selects it in `/workspace`.
+- **No AVX-512 hardware** in Cloud Agent VMs — SIMD kernel tests using `#[target_feature(enable = "avx512f")]` are compile-gated and will be skipped at runtime. This is expected behavior.
+- **Feature gates**: `intel-mkl` and `openblas` are mutually exclusive and require system libraries not installed by default. The default build uses `native` (pure Rust SIMD) which needs no extra libs.
+- **Build time**: ~18s cold, <1s incremental. Tests (~1819) take ~70s.
+- The workspace has sub-crates under `crates/` and `ndarray-rand/`. Default members exclude `blas-tests` and `blas-mock-tests` (they activate the `blas` feature which needs cblas-sys linking).
+- `libssl-dev` is needed as a build dependency for some transitive crates.