Adopt Home Manager; manage starship, nix.conf, and opencode config

install.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# Install this repo's system nix.conf into place. Idempotent.
+#
+#   sudo ./install.sh   update /etc/nix/nix.conf (backs up the current one)
+#
+# The user-level ~/.config/nix/nix.conf is managed declaratively by Home
+# Manager (see home/default.nix).
+
+set -euo pipefail
+
+repo_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+conf_file="$repo_dir/config/nix.conf"
+target=/etc/nix/nix.conf
+
+if [ ! -d "$(dirname "$target")" ]; then
+  sudo mkdir -p "$(dirname "$target")"
+fi
+if [ -f "$target" ] && ! cmp -s "$conf_file" "$target"; then
+  sudo cp -a "$target" "$target.bak.$(date +%Y%m%d%H%M%S)"
+fi
+sudo cp "$conf_file" "$target"
+echo "installed $target"