Port lib/opencode/rails/ source files; strip Rails.event/Rails.error

Eleven source files moved from ajent-rails:lib/opencode/rails/ to
opencode-rails:lib/opencode/ (flat layout — modules are Opencode::*, not
Opencode::Rails::*; matches opencode-ruby).

  artifact.rb           63 LOC
  exchange.rb           77 LOC
  impostor.rb           48 LOC
  message_artifacts.rb 133 LOC
  sandbox_file.rb       81 LOC
  sandbox.rb            71 LOC
  session.rb           168 LOC
  tool_display.rb      423 LOC
  transform.rb          77 LOC
  turn.rb              642 LOC
  uploaded_files_prompt.rb 85 LOC
  ----
  total              1,868 LOC

Surgical Rails strips:

  exchange.rb:
    Rails.event.notify(name, payload)
      -> Opencode::Instrumentation.instrument(name, payload) { }

  message_artifacts.rb (1 call), turn.rb (6 calls):
    Rails.error.report(error, **opts)
      -> Opencode::ErrorReporter.report(error, **opts)

Comments/docstrings referencing Rails.error.report / Rails.event left
in place — they document how to wire the host adapter.

ActiveSupport core_ext requires expanded in lib/opencode-rails.rb to
cover Numeric#seconds, Hash#deep_stringify_keys, String#squish/truncate,
String#demodulize. Bundle install + smoke load confirms all 12
gem-provided constants resolve cleanly.

lib/opencode-rails.rb

@@ -10,10 +10,12 @@
 
 require "opencode-ruby"
 
-require "active_support/core_ext/object/blank"
+require "active_support/core_ext/object/blank"      # blank?, present?, presence
 require "active_support/core_ext/object/try"
-require "active_support/core_ext/hash/keys"
-require "active_support/core_ext/string/inflections"
+require "active_support/core_ext/hash/keys"          # deep_stringify_keys, deep_symbolize_keys
+require "active_support/core_ext/string/inflections" # demodulize, underscore, camelize
+require "active_support/core_ext/string/filters"     # squish, truncate
+require "active_support/core_ext/numeric/time"       # 2.seconds, 5.minutes, etc.
 
 require_relative "opencode/rails/version"
 require_relative "opencode/error_reporter"