Loosen existing offline installs' exact Node pin (#13400)

5db4a401 fixed the installer, but machines already installed offline
still pin Node to the ISO's exact version, so mise up never updates it.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

migrations/1790457067.sh

@@ -0,0 +1,7 @@
+echo "Loosen the offline install's exact Node pin so mise up tracks new releases"
+
+mise_config="$HOME/.config/mise/config.toml"
+
+if [[ -f $mise_config ]] && grep -qE '^node = "[0-9]+\.[0-9]+\.[0-9]+"$' "$mise_config"; then
+  sed -i -E 's/^node = "[0-9]+\.[0-9]+\.[0-9]+"$/node = "latest"/' "$mise_config"
+fi