Use Opencode::Instrumentation.notify; bump dep + own version to alpha2

Paired-release commit that completes the .notify migration started in
opencode-ruby v0.0.1.alpha2.

Changes:
  - lib/opencode/exchange.rb: drops the empty '{ }' block from the
    apply_patch.artifacts_dropped emission, switching from
    .instrument(...) { } to .notify(...). Identical wire semantics,
    cleaner read at the call site.
  - opencode-rails.gemspec: pins opencode-ruby runtime dep to
    '= 0.0.1.alpha2' (was alpha1). Lockstep versions during alpha.
  - lib/opencode/rails_version.rb: 0.0.1.alpha1 -> 0.0.1.alpha2.
  - CHANGELOG: documents the Exchange call-site change and the
    opencode-ruby pin bump.

53 tests pass, 134 assertions, 0 failures.

lib/opencode/rails_version.rb

@@ -11,5 +11,5 @@
 # We can't reuse the same constant from a second gem, so we use a
 # distinct, non-namespaced constant.
 module Opencode
-  RAILS_VERSION = "0.0.1.alpha1"
+  RAILS_VERSION = "0.0.1.alpha2"
 end