[codex] Improve Nix flake and add direnv

[colonelpanic8]

Summary

• expose the development shell as devShells.default while preserving the legacy devShell output
• share GTK and system dependencies between the dev shell and package build
• derive the package version and metadata from Cargo.toml
• add .envrc so direnv can load the flake shell automatically

Validation

• alejandra flake.nix
• nix flake show --all-systems
• nix develop --command cargo check
• nix build
• nix run . -- --help

[cachebag]

LGTM