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.

lib/opencode/rails/version.rb

@@ -1,7 +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
-  module Rails
-    VERSION = "0.0.1.alpha1"
-  end
+  RAILS_VERSION = "0.0.1.alpha1"
 end