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/artifact.rb

@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+module Opencode
+  # A file the host wants to attach to an assistant message: filename,
+  # content bytes, MIME type, and an optional trust-metadata hash.
+  #
+  # Artifacts come from two places in the substrate:
+  #
+  #   - Opencode::Exchange.tool_artifacts  — content lives inside a tool
+  #     call's input/metadata (write tool).
+  #   - Opencode::SandboxFile#as_artifact — identity conversion of a
+  #     sandbox-resident file (the default path for Blackline + Raven).
+  #
+  # Transforms also return Artifacts; that's why FlightResultsTransform
+  # returns one with the host-rendered HTML + trust metadata stamp.
+  #
+  # An Artifact knows how to attach itself to a message, idempotently:
+  # it consults `message.artifacts` to skip if its filename is already
+  # there. The attaching verb belongs to the Artifact (the noun whose
+  # state the verb consults), not to a separate Attacher class.
+  class Artifact
+    attr_reader :filename, :content, :content_type, :metadata
+
+    def initialize(filename:, content:, content_type:, metadata: {})
+      @filename     = filename
+      @content      = content
+      @content_type = content_type
+      @metadata     = metadata
+    end
+
+    # Idempotent attach. Returns true if newly attached, false if the
+    # filename was already present on the message (so callers can count
+    # what they actually persisted vs what was already there).
+    def attach_to(message)
+      return false if already_attached_to?(message)
+
+      message.artifacts.attach(
+        io:           StringIO.new(content),
+        filename:     filename,
+        content_type: content_type,
+        metadata:     metadata
+      )
+      true
+    end
+
+    def already_attached_to?(message)
+      message.artifacts.any? { |a| a.filename.to_s == filename }
+    end
+
+    def ==(other)
+      other.is_a?(Artifact) &&
+        other.filename     == filename &&
+        other.content      == content &&
+        other.content_type == content_type &&
+        other.metadata     == metadata
+    end
+    alias_method :eql?, :==
+
+    def hash
+      [ filename, content, content_type, metadata ].hash
+    end
+  end
+end