feat(flake): add Nix development shell

[SDAChess]

Summary

Adds an initial Nix flake for the OpenShell development environment. The dev shell uses the repository rust-toolchain.toml through rust-overlay, and exposes nix fmt through treefmt-nix.

Related Issue

#1362

Changes

• Add flake.nix and flake.lock.
• Configure a default x86_64-linux, aarch64-linux, aarch64-darwin dev shell.
• Source the Rust toolchain from rust-toolchain.toml.
• Add pkg-config, libclang, and z3 for the Rust unit-test build.
• Configure treefmt-nix with nixfmt as the flake formatter.

Testing

• mise run pre-commit passes
• cargo test --workspace passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits

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[SDAChess]

/ok-to-test 4c2162a

[SDAChess]

/ok-to-test 95a4bcb

[elezar]

Thanks for these changes. I think they're low-risk enout to get in and iterate as required.