Support a persistent, private plugins directory alongside the built-in ones

plugins/ ships baked into the image and gets wiped/replaced by the
checkout sync on every deploy (docs/deployment.adoc), so a private
plugin dropped there — the exact use case the plugin contract was
designed for — didn't survive a redeploy the way uploaded GPX/kiwix
data under data/ does.

Plugins.list/run now accept either one directory or an array, and
app.rb checks the built-in plugins/ plus data/plugins/ (already
covered by the existing ./data bind mount, no new volume needed).

test/test_plugins.rb

@@ -52,4 +52,38 @@ class PluginsTest < Minitest::Test
     File.chmod(0o755, File.join(@plugin_dir, "run.sh"))
     assert_raises(Maptool::Plugins::RunError) { Maptool::Plugins.run(@dir, "hello", [1, 2, 3, 4]) }
   end
+
+  # Multiple dirs (app.rb's PLUGINS_DIRS: the built-in plugins/ plus an
+  # optional private one under data/plugins/) — list/run both accept an
+  # array, not just the single-dir form the tests above exercise.
+
+  def test_list_merges_plugins_across_several_dirs
+    other_dir = Dir.mktmpdir("maptool-plugins-test-2")
+    other_plugin_dir = File.join(other_dir, "private")
+    FileUtils.mkdir_p(other_plugin_dir)
+    File.write(File.join(other_plugin_dir, "plugin.json"), JSON.generate(label: "Private", command: "./run.sh"))
+
+    plugins = Maptool::Plugins.list([@dir, other_dir])
+
+    assert_equal [{ "name" => "hello", "label" => "Hello" }, { "name" => "private", "label" => "Private" }], plugins
+  ensure
+    FileUtils.remove_entry(other_dir)
+  end
+
+  def test_list_prefers_the_first_dir_when_the_same_name_exists_in_both
+    other_dir = Dir.mktmpdir("maptool-plugins-test-2")
+    FileUtils.mkdir_p(File.join(other_dir, "hello"))
+    File.write(File.join(other_dir, "hello", "plugin.json"), JSON.generate(label: "Shadowed", command: "./run.sh"))
+
+    plugins = Maptool::Plugins.list([@dir, other_dir])
+
+    assert_equal [{ "name" => "hello", "label" => "Hello" }], plugins
+  ensure
+    FileUtils.remove_entry(other_dir)
+  end
+
+  def test_run_finds_a_plugin_in_a_later_dir_when_earlier_dirs_do_not_have_it
+    result = Maptool::Plugins.run([Dir.mktmpdir, @dir], "hello", [1, 2, 3, 4])
+    assert_equal({ "type" => "FeatureCollection", "features" => [] }, result)
+  end
 end