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.

CHANGELOG.md

@@ -1,5 +1,21 @@
 # Changelog
 
+## 0.0.1.alpha2 — 2026-05-20
+
+### Changed
+
+- `Opencode::Exchange` now emits `opencode.apply_patch.artifacts_dropped`
+  via the new `Opencode::Instrumentation.notify` fire-and-forget API
+  (introduced in opencode-ruby v0.0.1.alpha2) instead of
+  `.instrument(name, payload) { }` with an empty block. Cleaner read
+  at the call site; identical semantics on the wire (same event name,
+  same payload).
+
+### Bumped
+
+- Runtime dependency `opencode-ruby` pinned to `= 0.0.1.alpha2` (was
+  `= 0.0.1.alpha1`). Versions stay in lockstep during alpha.
+
 ## 0.0.1.alpha1 — 2026-05-20
 
 Initial public alpha. Extracted from a production multi-product Rails app (`ajent-rails`) where these objects shipped under `lib/opencode/rails/` before being carved out into a standalone gem.