Better support GPG migrations

After yet another unpleasant experience starting up GPG on a new system, I
decided to encode my learnings and mistakes as aliases, functions, scripts,
hoping to protect my future me from myself. Fingers crossed!

configs/shared/gpg/.gnupg/export.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+set -e
+
+# Run this script to export all the information required to transport your GPG
+# information.
+# Usage: ./export.sh [directory]
+# TODO: run this periodically as a job.
+
+destination="${1:-$(mktemp -d)}"
+
+if [ ! -d "$destination" ]; then
+  echo "$destination does not exist. Creating it..."
+  mkdir -p "$destination"
+fi
+
+gpg --armor --export >"$destination/public.asc"
+gpg --armor --export-secret-keys >"$destination/secret.asc"
+gpg --armor --export-ownertrust >"$destination/ownertrust.txt"
+
+echo $(realpath "$destination")