Add a Nix flake for reproducible builds and dev shells

[maziyarpanahi]

Pull Request

Description

Adds a pinned Nix flake that builds the OpenMed Python package and provides a reproducible Python 3.12 development shell with the repository's development and test dependencies. A Linux CI workflow now builds the flake outputs and runs the full suite inside the shell.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test addition/improvement

Changes Made

• Add flake.nix and a committed flake.lock pinned to nixpkgs 26.05.
• Package OpenMed through buildPythonPackage with Hatchling and expose a development shell for all supported Linux and macOS architectures.
• Add a Linux flake validation workflow and document nix develop, nix build, checks, and intentional lock updates.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this change with different models/inputs

Validated with:

• nix flake check --print-build-logs
• nix build --no-link --print-build-logs
• nix flake show --all-systems
• nix develop --command python -m pytest tests/ -q (5,180 passed, 61 skipped)
• .venv/bin/python -m pytest tests/ -q (5,186 passed, 55 skipped)
• actionlint .github/workflows/nix.yml
• .venv/bin/python scripts/release/check_github_actions_refs.py
• uv run --extra docs mkdocs build --strict
• .venv/bin/pre-commit run --files .github/workflows/nix.yml docs/development.md flake.lock flake.nix mkdocs.yml

Documentation

• I have updated the documentation accordingly
• I have added docstrings to new functions/classes
• I have updated the CHANGELOG.md

Code Quality

• I ran make format, make lint, and make format-check
• For Swift/OpenMedKit changes, I ran make format-swift and make lint-swift
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

Dependencies

• I have not added any new dependencies
• OR I have added new dependencies and they are justified because: the flake pins nixpkgs as the reproducible Nix package source and CI installs Nix through a versioned action; Python runtime dependencies are unchanged.

Checklist

• I have read the contributing guidelines
• My commits have clear, descriptive messages
• I have squashed/organized my commits appropriately

Related Issues

Closes #1464

Screenshots/Examples

Not applicable; this change adds build, shell, CI, and documentation surfaces.