Code review actions: rename version file, drop Gemfile conditional, tighten gemspec, switch URLs to GitHub

Three tightly-coupled cleanups from the Tobi+Sandi review:

1. Rename lib/opencode/rails/version.rb -> lib/opencode/rails_version.rb
   (Sandi S1: file path was lying about its contents — the file defines
   Opencode::RAILS_VERSION, NOT Opencode::Rails::VERSION). Updated the
   require_relative in opencode-rails.gemspec and lib/opencode-rails.rb
   to match. Removed the now-empty lib/opencode/rails/ directory.

2. Drop the 'if File.exist?(...) gem opencode-ruby, path: ...' Gemfile
   conditional (Tobi T2 / Sandi S4: Bundler behavior must not depend on
   filesystem state). The dev-time sibling-repo override is now
   documented in CONTRIBUTING.md as the standard 'bundle config
   local.opencode-ruby <path>' pattern, which is what Bundler ships
   for this use case.

3. Tighten opencode-ruby runtime dep from '~> 0.0.1.alpha1' to
   '= 0.0.1.alpha1' (Tobi T1: ~> during alpha is aspirational; pin
   exactly until the public API stabilizes).

Same commit also switches all forward-looking URLs (gemspec homepage,
metadata, README link to opencode-ruby) from Gitea to GitHub since the
gems will eventually publish there. Functional 'git:' URL in
ajent-rails' Gemfile stays on Gitea — that's where the gems actually
are right now; ajent-rails Gemfile flips to GitHub when the user does
the actual remote setup.

Test assertion in loading_test was tightened to match either an
installed-gem path ('gems/opencode-X-VERSION/') or a sibling-repo
checkout ('/opencode-X/'), via a small GEM_PATH_PATTERN helper.

15 tests, 50 assertions, 0 failures.

Gemfile

@@ -1,11 +1,3 @@
 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