Rename Opencode::Rails::VERSION -> Opencode::RAILS_VERSION

Critical fix: defining Opencode::Rails as a module shadowed ::Rails
under Ruby's constant lookup whenever host code referenced top-level
Rails.something from inside the Opencode:: namespace.

Caught when host code in lib/opencode/containers/container.rb failed
to boot:

  /workspaces/app/lib/opencode/containers/container.rb:433:
    undefined method 'root' for module Opencode::Rails (NoMethodError)
      "notes.md" => Rails.root.join(...)

Ruby resolved 'Rails' to 'Opencode::Rails' first (the gem's version
namespace) before falling back to ::Rails. The fix removes the
intermediate module entirely:

  module Opencode
    RAILS_VERSION = "0.0.1.alpha1"
  end

Added a regression test (test_no_opencode_rails_module) so the
shadowing never sneaks back in. opencode-ruby uses Opencode::VERSION
for its own gem version; we can't double-up on that constant, so this
gem uses RAILS_VERSION as a sibling on the same Opencode module.

opencode-rails.gemspec

@@ -4,7 +4,7 @@ require_relative "lib/opencode/rails/version"
 
 Gem::Specification.new do |spec|
   spec.name          = "opencode-rails"
-  spec.version       = Opencode::Rails::VERSION
+  spec.version       = Opencode::RAILS_VERSION
   spec.authors       = ["Ajay Krishnan"]
   spec.email         = ["ajay@krishnan.ca"]