Add Opencode::Instrumentation.notify; bump to 0.0.1.alpha2

Code review consensus from Tobi+Sandi: the empty-block call
'Opencode::Instrumentation.instrument(name, payload) { }' at fire-
and-forget call sites in opencode-rails is API smell. Tobi: 'two
named verbs are clearer than one verb with a vestigial block.'
Sandi: 'a method with a block parameter that's optional but expected
empty in some call sites is doing two things.'

Two emission shapes now:

  .instrument(name, payload) { ... }   # block; duration measured
  .notify(name, payload)                # fire-and-forget; no block

Both flow through the same adapter. The adapter still always
receives a block argument (some adapters key on it, e.g. AS::
Notifications.instrument requires a block) — .notify passes an
empty {}. Adapter return value is ignored for .notify (it returns
nil); .instrument continues to pass through the block's return.

Three new tests in smoke_test.rb:
  - no-op when no adapter set
  - forwards to adapter + verifies block presence + verifies that
    .notify returns nil (not the adapter's return)
  - works without a block at the call site

Also: switched gemspec metadata URLs from Gitea to GitHub. The gem
will eventually publish from github.com/ajaynomics/opencode-ruby —
the metadata now reflects that. (No actual GitHub remote push yet;
that's the user's manual step.)

15 tests pass, 32 assertions, 0 failures.

CHANGELOG.md

@@ -1,5 +1,23 @@
 # Changelog
 
+## 0.0.1.alpha2 — 2026-05-20
+
+### Added
+
+- `Opencode::Instrumentation.notify(name, payload)` — fire-and-forget
+  emission for point-in-time events that don't need duration measurement
+  (apply_patch.artifacts_dropped, session.recreated, etc.). Adapter
+  receives an empty block so AS::Notifications-shaped sinks see a
+  zero-duration event. Complements the existing block-form
+  `.instrument(name, payload) { ... }`.
+
+### Why
+
+The block-form `.instrument(name, payload) { }` with an empty block was
+awkward at fire-and-forget call sites in opencode-rails. Two named
+verbs (`instrument` for wrap-a-block, `notify` for fire-and-forget)
+match the host-side mental model and read better at the call site.
+
 ## 0.0.1.alpha1 — Unreleased
 
 First public alpha. HTTP + SSE client for OpenCode REST API.