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.

opencode-ruby.gemspec

@@ -16,12 +16,13 @@ Gem::Specification.new do |spec|
     OpenAPI) — pick this one if you want a small Ruby-idiomatic surface;
     pick opencode_client if you want every endpoint with generated types.
   DESC
-  spec.homepage      = "https://gitea.krishnan.ca/ajaynomics/opencode-ruby"
+  spec.homepage      = "https://github.com/ajaynomics/opencode-ruby"
   spec.license       = "MIT"
   spec.required_ruby_version = ">= 3.2.0"
 
+  spec.metadata["homepage_uri"]    = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage
-  spec.metadata["changelog_uri"]   = "#{spec.homepage}/src/branch/main/CHANGELOG.md"
+  spec.metadata["changelog_uri"]   = "#{spec.homepage}/blob/main/CHANGELOG.md"
   spec.metadata["bug_tracker_uri"] = "#{spec.homepage}/issues"
 
   spec.files = Dir.glob("lib/**/*.rb") +