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.

Gemfile

@@ -0,0 +1,11 @@
+source "https://rubygems.org"
+
+# opencode-ruby is the underlying wire client. During alpha both gems
+# evolve in lockstep; the gemspec pins a release version, but for local
+# dev we pull from the sibling working copy so changes in opencode-ruby
+# are picked up without a release cycle.
+if File.exist?(File.expand_path("../opencode-ruby", __dir__))
+  gem "opencode-ruby", path: File.expand_path("../opencode-ruby", __dir__)
+end
+
+gemspec