Replace Nix flake-based development setup with devenv for better
developer experience and more streamlined environment management.
Changes:
- Remove flake.nix and flake.lock files
- Add devenv.nix, devenv.yaml, and devenv.lock configuration
- Update .envrc to use devenv instead of nix develop
- Remove Docker development setup (compose.dev.yml, docker/mod.just)
- Expand .gitignore with comprehensive IDE and OS exclusions
- Remove Docker-related just commands from justfile
