Scaffold opencode-rails gem structure

- opencode-rails.gemspec (Ruby >= 3.2, MIT)
  - runtime: opencode-ruby ~> 0.0.1.alpha1
  - runtime: activerecord/activestorage/activesupport >= 7.1, < 9.0
    Per-sublibrary deps (not the rails umbrella) so host apps don't
    pull in ActionMailer/ActionCable/ActionView just to use this gem.
- lib/opencode-rails.rb (umbrella; requires opencode-ruby then leaves
  then session/turn)
- lib/opencode/rails/version.rb (Opencode::Rails::VERSION = 0.0.1.alpha1)
- Gemfile (sibling-path pull of opencode-ruby for local dev)
- Rakefile, .gitignore

Source files will be ported in the next commit.

Rakefile

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+require "bundler/gem_tasks"
+require "rake/testtask"
+
+Rake::TestTask.new(:test) do |t|
+  t.libs << "test"
+  t.libs << "lib"
+  t.test_files = FileList["test/**/*_test.rb"]
+end
+
+task default: :test