Auto-install .NET SDK in CI scripts when runner lacks it

Gitea runner build failed with "dotnet: command not found". Added
ensure_dotnet() to lib.sh, using the vendored dotnet-install.sh to
bootstrap the SDK into .dotnet/ when not already on PATH, called from
build.sh/test.sh/prepush.sh. Also trigger CI on build-runner-fix to
verify the fix.

scripts/ci/lib.sh

@@ -37,6 +37,23 @@ image_names() {
   ADMIN_IMAGE="$REGISTRY/$REGISTRY_OWNER/miccheck-admin"
 }
 
+# Installs .NET into $CI_ROOT/.dotnet via the vendored dotnet-install.sh if
+# `dotnet` isn't already on PATH, then prepends it to PATH for this process.
+# Keeps bare runners (no SDK preinstalled) working the same as a dev machine.
+ensure_dotnet() {
+  if command -v dotnet > /dev/null 2>&1; then
+    return 0
+  fi
+
+  local install_dir="$CI_ROOT/.dotnet"
+  if [[ ! -x "$install_dir/dotnet" ]]; then
+    log "dotnet not found on PATH; installing .NET SDK via dotnet-install.sh"
+    bash "$CI_ROOT/dotnet-install.sh" --channel LTS --install-dir "$install_dir"
+  fi
+  export PATH="$install_dir:$PATH"
+  export DOTNET_ROOT="$install_dir"
+}
+
 registry_login() {
   require_registry_vars
   [[ -n "$REGISTRY_USER" ]] || fail "REGISTRY_USER env var is required to push images"