feat: retool setup scripts as Debian 13-first (keep Fedora support)

- main-setup.sh: sudo/root preflight for fresh Debian installs; offer
  distro-agnostic 03-dotnet-setup.sh on all supported distros
- 01-package-install.sh: Debian package renames (bind9-dnsutils), batcat
  symlink, libvirt/QEMU stack; vendor repos for gh/terraform/kubectl +
  k9s/minikube binaries (not in Debian main); keep dnf path for Fedora
- 02-dev-tools-setup.sh: PEP 668 fallback for pynvim pip install
- 03-fedora-dotnet-setup.sh -> 03-dotnet-setup.sh: .NET SDK via official
  dotnet-install.sh (no MS repo), PostgreSQL + Podman/podman-docker
- zshrc: purge dead oh-my-zsh references (prompt is Oh-My-Posh)
- README: Debian 13 first-class; docs: record decisions + status

zshrc

@@ -1,10 +1,6 @@
-# Path to your oh-my-zsh installation.
-export ZSH=$HOME/.oh-my-zsh
-
+# Plain zsh config (oh-my-zsh was removed in favor of Oh-My-Posh).
 DEFAULT_USER="$USER"
 
-plugins=(git)
-
 # Editor
 export EDITOR='nvim'
 
@@ -23,7 +19,6 @@ export NVM_DIR="$HOME/.nvm"
 # Rust/Cargo
 [ -f "$HOME/.cargo/env" ] && source "$HOME/.cargo/env"
 
-source $ZSH/oh-my-zsh.sh
 source $HOME/dotfiles/aliases.zsh
 
 # SSH agent
@@ -43,3 +38,5 @@ export CPLUS_INCLUDE_PATH=/usr/local/include
 
 # Oh My Posh prompt
 eval "$(oh-my-posh init zsh --config ~/.config/oh-my-posh/theme.omp.json)"
+# added by 02-dev-tools-setup.sh
+export PATH=$PATH:/usr/local/go/bin