Publish provider binaries in the release-plz release flow

[Teakowa]

Goal

Publish the compiled first-party opy-provider binaries and checksums as assets of the GitHub Release created by the same release-plz run.

Context

opy-provider is a repository-specific executable distributed alongside opy-rs releases for Wright's first-party provider resolver. The current implementation builds provider binaries in a separate tag-triggered workflow and then rediscovers/waits for the GitHub Release before uploading assets.

That creates an unnecessary second release lifecycle. release-plz already creates the tag and GitHub Release, and its action exposes whether a release was created plus the corresponding release metadata. The shared WrightKit release workflow should expose that generic information to callers; opy-rs can then build and attach its repository-specific binaries as downstream jobs of the same release flow.

Scope

• Consume the release result/metadata exposed by the shared release-plz workflow.
• When the opy-rs release is created, build the supported opy-provider target matrix from that release revision.
• Attach each provider archive and matching .sha256 file to the GitHub Release created by that release-plz run.
• Remove the separate tag-triggered provider release lifecycle once the integrated flow replaces it.
• Preserve the asset naming contract consumed by Wright's first-party provider resolver.

Non-goals

• No provider runtime or LPP changes.
• No OPY compiler/semantic changes.
• No Wright dependency or provider-resolution changes.
• No provider-specific logic in the shared .github reusable workflow.
• No independent tag/release polling or rediscovery protocol.
• No redesign of release-plz versioning or crate publication.

Acceptance criteria

• Provider binary publication is downstream of the same release-plz invocation that creates the opy-rs GitHub Release.
• No separate push.tags / release workflow is required solely to attach provider assets.
• No polling or fixed-delay wait is used to synchronize with GitHub Release creation.
• A release builds every supported provider target and attaches each archive plus its matching .sha256 asset to that release.
• Published asset names remain compatible with Wright's first-party provider resolver.
• Runs where release-plz creates no release do not build or publish provider release assets.
• A subsequent release can repeat the complete flow without manual intervention.

Dependencies / ownership

• Depends on Expose release-plz release outputs to reusable workflow callers .github#34 for the generic reusable-workflow release outputs.
• wrightkit/.github owns only the reusable release-plz output contract.
• opy-rs owns provider build, packaging, asset naming, and publication.

Refs #170
Refs #175