Add Nix flake + GitHub Actions CI and bump to v0.1.0

[crertel]

Motivation

• Provide a reproducible Nix development shell and a simple, reproducible CI entrypoint so contributors can run the same checks locally and in CI.
• Add GitHub Actions workflows to run Nix-based checks on pushes and pull requests.
• Bump the package version to reflect the new development/CI improvements.

Description

• Expanded flake.nix from a dev-shell-only file into a development+CI flake that exposes a default dev shell, a ci flake app (nix run .#ci) that runs mix deps.get, mix format --check-formatted, and mix test, and a formatter output (nixpkgs-fmt).
• Added .github/workflows/ci.yml to run project checks inside Nix (nix develop --command nix run .#ci) on push and pull_request to main/master.
• Added .github/workflows/flake.yml to run nix flake check on push and pull_request to main/master.
• Bumped library version in mix.exs from 0.0.1 to 0.1.0 and updated README.md to show the new GitHub Actions CI badge and a local command to run CI checks (nix run .#ci).

Testing

• Ran mix format --check-formatted, which completed successfully.
• Attempted mix test, which could not complete due to inability to install Hex in this environment (HTTP 403 tunnel restriction) so tests were blocked.
• Attempted to exercise Nix flows (nix flake lock / nix develop -c mix test), which failed to fetch GitHub flake inputs in this environment due to HTTP 403 tunnel restrictions.

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Codex Task