Add Opencode::ErrorReporter pluggable adapter

Per the same pattern as Opencode::Instrumentation in opencode-ruby:
zero-dependency default (no-op), host plugs an adapter in to route
errors to Rails.error / Honeybadger / Sentry / etc.

Used by Session, Turn, MessageArtifacts (ported in the next commit) to
report swallowed exceptions and degraded paths without coupling the
gem to any specific error-tracking library.

  # config/initializers/opencode.rb
  Opencode::ErrorReporter.adapter = ->(error, **opts) {
    Rails.error.report(error, **opts)
  }

lib/opencode/error_reporter.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+module Opencode
+  # Pluggable error-reporter adapter. opencode-rails ships with zero
+  # dependency on Honeybadger, Sentry, Bugsnag, Rollbar, or any specific
+  # error-tracking library. Host apps plug their own adapter in:
+  #
+  #   # config/initializers/opencode.rb
+  #   Opencode::ErrorReporter.adapter = ->(error, **opts) {
+  #     Rails.error.report(error, **opts)
+  #   }
+  #
+  # When no adapter is set (default), `.report` is a silent no-op. This
+  # lets the gem be used outside Rails-style apps without forcing a
+  # dependency on any specific error reporter, while keeping the option
+  # to route errors anywhere the host wants when the gem ships in a
+  # Rails context.
+  #
+  # Semantics mirror Rails.error.report (handled:, severity:, context:)
+  # but the gem doesn't enforce any specific kwarg vocabulary — whatever
+  # the gem code passes is forwarded verbatim to the adapter.
+  module ErrorReporter
+    class << self
+      attr_accessor :adapter
+    end
+
+    # Report an error to the configured adapter, or no-op if none set.
+    # Returns the adapter's return value (typically the error itself for
+    # `Rails.error.report`).
+    def self.report(error, **opts)
+      adapter&.call(error, **opts)
+    end
+  end
+end