Improve nix flake

[krisztianfekete]

This PR improves the flake.nix to follow best practices:

• Removed unused devshell input
• Fixed warning from flake-utils having a nixpkgs follows it doesn't need
• Added packages.default (full venv) and packages.agentevals-cli (bare package) so nix build also works
• Added apps.default so nix run . launches the CLI directly
• Added formatter output for nix fmt support
• Small Nix idiom cleanups

New capabilities:

• nix build, produces a self-contained virtualenv with the agentevals CLI
• nix build .#agentevals-cli, builds just the Python package
• nix run ., runs agentevals directly
• nix fmt, formats the flake with nixfmt-rfc-style
• nix develop, same devShell as before