Add schema inspection queries and script.

SPARQL queries to list classes and properties, and src/schema.rb
CLI to run them against any graph file. Includes bundler/setup
for compatibility with Ruby 4.0 without bundle exec.

queries/list_classes.sparql

@@ -0,0 +1,4 @@
+SELECT DISTINCT ?class WHERE {
+  ?s a ?class .
+}
+ORDER BY ?class

queries/list_properties.sparql

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+SELECT DISTINCT ?property WHERE {
+  ?s ?property ?o .
+  FILTER(?property != <http://www.w3.org/1999/02/22-rdf-syntax-ns#type>)
+}
+ORDER BY ?property

src/schema.rb

@@ -0,0 +1,40 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+# Usage: ruby src/schema.rb TYPE [--graph FILE]
+#   TYPE: classes | properties
+#   FILE: path to an RDF/Turtle file (default: last data/graph-*.ttl)
+
+require 'bundler/setup'
+require 'rdf'
+require 'rdf/turtle'
+require 'sparql'
+
+type = ARGV.shift
+if !%w[classes properties].include?(type)
+  abort "Usage: ruby src/schema.rb TYPE [--graph FILE]\n  TYPE: classes | properties"
+end
+
+graph_file = nil
+if ARGV[0] == '--graph'
+  ARGV.shift
+  graph_file = ARGV.shift
+end
+
+graph_file ||= Dir.glob(File.join('data', 'graph-*.ttl')).sort.last
+
+abort "No graph file found." if graph_file.nil? || !File.exist?(graph_file)
+
+query_file = File.join(__dir__, '..', 'queries', "list_#{type}.sparql")
+query = File.read(query_file)
+
+$stderr.puts "Loading #{graph_file}..."
+graph = RDF::Graph.load(graph_file)
+$stderr.puts "Loaded #{graph.count} triples."
+
+solutions = SPARQL.execute(query, graph)
+
+key = type == 'classes' ? :class : :property
+solutions.each do |solution|
+  puts solution[key].to_s
+end