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.

lib/opencode/rails_version.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+# NOTE: we deliberately do NOT define `Opencode::Rails` as a module —
+# host applications often have files inside the `Opencode::` namespace
+# that reference top-level `::Rails.something`. Defining
+# `Opencode::Rails` would shadow `::Rails` under Ruby's constant
+# lookup rules (`Rails.root` would resolve to `Opencode::Rails.root`
+# and raise NoMethodError).
+#
+# The opencode-ruby gem uses `Opencode::VERSION` for its own version.
+# We can't reuse the same constant from a second gem, so we use a
+# distinct, non-namespaced constant.
+module Opencode
+  RAILS_VERSION = "0.0.1.alpha1"
+end