feat: add nvm setup script for CI runners

Installs nvm, Node.js LTS, and creates a stable symlink at
~/.nvm/default_bin for the systemd PATH so actions/checkout@v4
can find node without sourcing .bashrc.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

script/setup/nvm.sh

@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+# Set up nvm and a stable default Node.js for a gitea_runner user on a CI runner.
+#
+# Usage: ./script/setup/nvm.sh <runner-host>
+#
+# This script:
+#   1. Installs nvm for the gitea_runner user (if not already present)
+#   2. Installs the current Node.js LTS version
+#   3. Creates a stable symlink at ~/.nvm/default_bin pointing to the
+#      default node binary directory
+#   4. Prints the systemd Environment line needed for the runner unit
+#
+# After running, add the following to the runner's systemd unit
+# (/etc/systemd/system/gitea-action-runner.service):
+#
+#   Environment=PATH=/var/lib/gitea_runner/.nvm/default_bin:/usr/local/sbin:/usr/local/bin:/usr/bin
+#
+# Then: sudo systemctl daemon-reload && sudo systemctl restart gitea-action-runner
+
+set -euo pipefail
+
+runner_host="${1:?usage: $0 <runner-host>}"
+runner_user="gitea_runner"
+
+echo "installing nvm on ${runner_host} for ${runner_user}..."
+ssh "${runner_host}" "sudo -u ${runner_user} bash -c '
+    export NVM_DIR=\"\${HOME}/.nvm\"
+    if [ -s \"\${NVM_DIR}/nvm.sh\" ]; then
+        echo \"nvm already installed at \${NVM_DIR}\"
+    else
+        curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.40.3/install.sh | bash
+    fi
+'"
+
+echo "installing Node.js LTS and creating stable symlink..."
+ssh "${runner_host}" "sudo -u ${runner_user} bash -c '
+    export NVM_DIR=\"\${HOME}/.nvm\"
+    . \"\${NVM_DIR}/nvm.sh\"
+    nvm install --lts
+    nvm alias default lts/*
+    ln -sfn \"\$(dirname \"\$(nvm which default)\")\" \"\${NVM_DIR}/default_bin\"
+    echo \"node: \$(node --version)\"
+    echo \"symlink: \$(readlink -f \"\${NVM_DIR}/default_bin\")\"
+'"
+
+echo ""
+echo "add the following to /etc/systemd/system/gitea-action-runner.service on ${runner_host}:"
+echo ""
+echo "  Environment=PATH=/var/lib/${runner_user}/.nvm/default_bin:/usr/local/sbin:/usr/local/bin:/usr/bin"
+echo ""
+echo "then run:"
+echo "  ssh ${runner_host} 'sudo systemctl daemon-reload && sudo systemctl restart gitea-action-runner'"
+echo ""
+echo "also add 'nvm' to the runner's labels in /etc/act_runner/config.yml"