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.

test/opencode/loading_test.rb

@@ -49,6 +49,16 @@ class Opencode::LoadingTest < Minitest::Test
   end
 
   def test_version_constant
-    assert_match(/\A\d+\.\d+\.\d+/, Opencode::Rails::VERSION)
+    assert_match(/\A\d+\.\d+\.\d+/, Opencode::RAILS_VERSION)
+  end
+
+  def test_no_opencode_rails_module
+    # Defining Opencode::Rails as a module would shadow ::Rails for any
+    # host code that references top-level Rails.* from inside the
+    # Opencode:: namespace (e.g. lib/opencode/containers/container.rb).
+    # Verify the namespace stays clean — version lives at
+    # Opencode::RAILS_VERSION, not Opencode::Rails::VERSION.
+    refute Opencode.const_defined?(:Rails),
+      "Opencode::Rails must not be defined — it would shadow ::Rails inside the Opencode namespace"
   end
 end