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/uploaded_files_prompt.rb

@@ -0,0 +1,85 @@
+# frozen_string_literal: true
+
+module Opencode
+  # The prompt body to send to an OpenCode agent when the user attached
+  # files: the user's text plus an instruction block naming each file by
+  # its sandboxed filename so the agent can read it with the `read` tool.
+  #
+  # Two outputs, both explicit:
+  #
+  #   text                  — the prompt body to pass to send_message_async
+  #   sandbox_file_names    — map of sandbox_name => original filename,
+  #                           used by ReplyStream to show the user a
+  #                           recognizable name when the agent reads the
+  #                           file back.
+  #
+  # Previously this work lived in `Opencode::SandboxFiles`, an ActiveSupport
+  # concern that mutated a hidden `@sandbox_file_names` instance variable on
+  # the including job. ReplyStream then read that ivar back through a
+  # closure. State across class boundaries via shared mutable ivars is the
+  # kind of Sandi-smelly action-at-a-distance that breaks the moment
+  # someone forgets the contract. This value object replaces that with two
+  # named return values.
+  #
+  # Side effect, unchanged from the concern: file bytes are copied from
+  # ActiveStorage into the per-user OpenCode sandbox directory so the
+  # agent can read them with the `read` tool. The copy is path-escape
+  # guarded (the cleanpath of the destination must start with the
+  # sandbox dir prefix, no symlink trickery).
+  class UploadedFilesPrompt
+    attr_reader :text, :sandbox_file_names
+
+    def initialize(user_message:, sandbox_path:, sandbox_name_for:)
+      @user_message = user_message
+      @sandbox_path = sandbox_path
+      @sandbox_name_for = sandbox_name_for
+      @sandbox_file_names = {}
+      @text = build_text
+    end
+
+    private
+
+    def build_text
+      raw = @user_message.content.to_s
+      return raw unless @user_message.files.attached?
+
+      file_instructions = @user_message.files.map do |file|
+        sandbox_file = copy_to_sandbox(file)
+        @sandbox_file_names[sandbox_file.sandbox_name] = file.filename.to_s
+        "#{file.filename} -> #{sandbox_file.sandbox_name} (#{file.content_type}, #{file.byte_size} bytes)"
+      end
+
+      [
+        raw,
+        "",
+        "The user uploaded #{file_instructions.size} file(s). Read each file thoroughly, then consult your reference materials and verify any legal claims before responding:",
+        *file_instructions
+      ].join("\n").strip
+    end
+
+    def copy_to_sandbox(file)
+      FileUtils.mkdir_p(@sandbox_path)
+
+      sandbox_name = @sandbox_name_for.call(file)
+      dest = File.join(@sandbox_path, sandbox_name)
+
+      resolved = Pathname.new(dest).cleanpath.to_s
+      unless resolved.start_with?(@sandbox_path)
+        raise ArgumentError, "Filename escapes sandbox: #{sandbox_name}"
+      end
+
+      File.open(dest, "wb") { |f| f.write(file.download) }
+      Placement.new(sandbox_name, dest)
+    end
+
+    # Tiny value pair returned by copy_to_sandbox: the canonical filename
+    # the agent should read by, and the on-disk path the file ended up at.
+    # Internal to UploadedFilesPrompt — the caller (UploadedFilesPrompt
+    # itself) only needs the sandbox_name to embed in the prompt text.
+    Placement = Struct.new(:sandbox_name, :path) do
+      def to_s
+        path
+      end
+    end
+  end
+end