feat: add Nix flake for declarative builds and development

[smantzavinos]

Summary

• Add Nix flake support for declarative builds and development environments
• Enables nix build, nix run, and nix develop for the project

Changes

• flake.nix: Nix flake with buildNpmPackage, devShell, and app outputs
• flake.lock: Locked to nixos-25.11 stable
• .gitignore: Added result and .direnv/ entries
• README.md: Added Nix installation option in collapsible section

Usage

# Install globally
nix profile install github:numman-ali/openskills

# Run directly without installing
nix run github:numman-ali/openskills -- --help

# Development shell
nix develop github:numman-ali/openskills