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

scripts/02-dev-tools-setup.sh

@@ -35,12 +35,19 @@ else
 fi
 
 # pynvim
+# On Debian 13+/Ubuntu 23.04+ the system python is PEP 668 'externally-managed',
+# so a plain `pip install --user` is rejected. Fall back to --break-system-packages.
 echo "Installing pynvim for Neovim Python support..."
 if /usr/bin/python3 -m pip show pynvim &>/dev/null; then
     echo "pynvim already installed."
 else
-    /usr/bin/python3 -m pip install --user pynvim
-    echo "pynvim installed for the current user."
+    if /usr/bin/python3 -m pip install --user pynvim; then
+        echo "pynvim installed for the current user."
+    elif /usr/bin/python3 -m pip install --user --break-system-packages pynvim; then
+        echo "pynvim installed for the current user (PEP 668 --break-system-packages fallback)."
+    else
+        echo "WARNING: Could not install pynvim. Install it inside your Neovim Python provider venv manually."
+    fi
 fi
 
 # Install Nerd Font (Hack)