Add GitHub Release binary asset publishing

[Copilot]

GitHub Releases were missing downloadable binary archives — users had no first-class binary artifact channel without going through crates.io or npm.

Changes

• New publish-github-release-assets job in .github/workflows/release.yml
  • Gated on both publish-crates and publish-npm succeeding; skipped if either fails
  • Downloads all five build matrix artifacts, packages them as archives, and uploads to the GitHub Release
  • permissions: contents: write scoped to this job only

Assets uploaded per release

File	Target
99problems-<ver>-x86_64-pc-windows-msvc.zip	Windows x64
99problems-<ver>-x86_64-unknown-linux-gnu.tar.gz	Linux x64
99problems-<ver>-x86_64-apple-darwin.tar.gz	macOS x64
99problems-<ver>-aarch64-apple-darwin.tar.gz	macOS arm64
99problems-<ver>-aarch64-unknown-linux-gnu.tar.gz	Linux arm64
SHA256SUMS.txt	Checksum manifest

Uses gh release upload --clobber so workflow reruns replace assets cleanly without manual cleanup.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Feature] Publish GitHub release binary assets</issue_title>
<issue_description>### Problem

Release builds already produce platform binaries and publish crates/npm packages, but the GitHub Release itself does not provide downloadable binary assets.

Users who want direct download from GitHub Releases cannot use a first-class binary artifact channel.

Design

Add a final release-assets job that:

1. runs only after publish-crates and publish-npm succeed,
2. downloads existing build artifacts,
3. packages per-target archives,
4. uploads archives + checksums to the GitHub Release for the tag.

Scope

• Extend .github/workflows/release.yml with a gated publish-github-release-assets job.
• Require both registry publish jobs to succeed before release asset upload.
• Upload these assets:
  • Windows zip archive
  • Linux/macOS tar.gz archives
  • SHA256SUMS.txt

Boundary

• No changes to provider/source code.
• No change to build target matrix in this issue.
• No code signing/notarization in this issue.

Acceptance Criteria

• On v* release tag workflow:
  • crates.io publish succeeds,
  • npm publish succeeds,
  • then (and only then) release assets are attached to the GitHub Release.
• If either registry publish fails, the release-assets job does not run.
• Release contains archives for all currently supported targets and a checksum manifest.
• Workflow rerun keeps assets consistent and usable.

Context

This improves release usability for direct binary consumers while preserving strict release ordering and existing registry-first publication policy.

• GitHub Actions workflow syntax: https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions
• GitHub Actions events (tag push): https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#push
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Feature] Publish GitHub release binary assets #68

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