Add smoke tests for opencode-rails

14 tests across 4 files covering the gem's public surface:

  test/opencode/loading_test.rb (5 tests)
    - All 12 gem-provided constants resolve after require
    - Transitively-loaded opencode-ruby constants are present
    - Opencode::Session source_location points at this gem
    - Opencode::Client source_location points at opencode-ruby
    - Opencode::Rails::VERSION matches semver

  test/opencode/error_reporter_test.rb (3 tests)
    - report is no-op without adapter (returns nil)
    - report forwards error + opts to adapter
    - report works with no kwargs

  test/opencode/exchange_test.rb (3 tests)
    - Initializes with empty / nil / Array(coerced) messages
    - tool_artifacts(exclude:) doesn't raise on empty input

  test/opencode/artifact_test.rb (3 tests)
    - Filename/content/content_type/metadata readers
    - metadata defaults to {} when omitted
    - Trust metadata round-trips

Behavioral tests for Session/Turn/MessageArtifacts (AR + ActiveStorage
fixtures) stay in the host application (ajent-rails) where the
integration environment exists. Same pattern as opencode-ruby:
  - gem smoke tests prove load-time correctness
  - host tests prove integration correctness

test/opencode/artifact_test.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+# Smoke test for Opencode::Artifact — verifies the value-object surface
+# (filename/content/content_type readers). Behavioral tests around how
+# the host's ActiveStorage-backed AIGL::Trip/etc. records build artifact
+# collections live in the host's test suite.
+class Opencode::ArtifactTest < Minitest::Test
+  def test_value_object_readers
+    artifact = Opencode::Artifact.new(
+      filename: "notes.md",
+      content: "# Notes",
+      content_type: "text/markdown"
+    )
+
+    assert_equal "notes.md", artifact.filename
+    assert_equal "# Notes", artifact.content
+    assert_equal "text/markdown", artifact.content_type
+  end
+
+  def test_metadata_defaults_to_empty_hash
+    artifact = Opencode::Artifact.new(filename: "x.txt", content: "hi", content_type: "text/plain")
+    assert_equal({}, artifact.metadata)
+  end
+
+  def test_metadata_accepts_trust_hash
+    artifact = Opencode::Artifact.new(
+      filename: "x.html",
+      content: "<p>",
+      content_type: "text/html",
+      metadata: { trust: "host_rendered" }
+    )
+    assert_equal "host_rendered", artifact.metadata[:trust]
+  end
+end