feat: unified shell + sandbox setup with Copier, devcontainer CLI, and robust features

[jonmatum]

• Added cross-distro shell setup with Zsh, Oh My Zsh, Powerlevel10k, autosuggestions, syntax-highlighting
• Automatic Meslo Nerd Font v3 installation with fallback and cache refresh
• Cross-platform timezone configuration with tzdata validation and install
• Support for opinionated .zshrc and .p10k.zsh with override URLs
• Post-install script hook with URL-based injection
• Added final status summary CLI to verify key tools (terraform, zsh, aws, etc.)
• Introduced Make + Copier-based workflow for sandbox generation
• Interactive feature/image selection with persistent answer file
• Auto-installs copier in isolated venv and injects features into sandbox
• Format/validate devcontainer.json using jq and python tools
• Enhanced Make targets: sandbox, devcontainer-up, exec, interactive, help
• Added validation, fallback behavior, and robust logging