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.

test/opencode/loading_test.rb

@@ -36,16 +36,24 @@ class Opencode::LoadingTest < Minitest::Test
     end
   end
 
+  # We check via path match on both directory ("/opencode-rails/") and
+  # installed-gem name ("/opencode-rails-VERSION/") so the assertion is
+  # robust to either a sibling-repo dev setup or a bundle-resolved gem
+  # install.
+  GEM_PATH_PATTERN = ->(name) { %r{/#{Regexp.escape(name)}[-/]} }
+
   def test_session_constant_points_at_this_gem
     location = Opencode::Session.instance_method(:initialize).source_location.first
-    assert_match %r{/opencode-rails/}, location,
+    assert_match GEM_PATH_PATTERN.call("opencode-rails"), location,
       "Expected Opencode::Session to be loaded from opencode-rails, got: #{location}"
   end
 
   def test_client_constant_points_at_opencode_ruby
     location = Opencode::Client.instance_method(:initialize).source_location.first
-    assert_match %r{/opencode-ruby/}, location,
+    assert_match GEM_PATH_PATTERN.call("opencode-ruby"), location,
       "Expected Opencode::Client to come from opencode-ruby, got: #{location}"
+    refute_match GEM_PATH_PATTERN.call("opencode-rails"), location,
+      "Opencode::Client must NOT come from opencode-rails (it's an opencode-ruby class)"
   end
 
   def test_version_constant