Add release health manifest generation to release workflow

[jetyu]

Motivation

• Ensure a machine-readable health.json is produced for each release so consumers (GitHub assets and Cloudflare R2) can verify release health before serving updates.
• Verify the presence and non-empty contents of three platform manifests (latest.yml, latest-mac.yml, latest-linux.yml) and surface a failure early if any are missing.
• Attach the generated health manifest to existing release artifacts so it is uploaded to both GitHub Releases and the Cloudflare R2 channel directories.

Description

• Added a Generate release health manifest step to .github/workflows/build.yml that runs before the GitHub release page generation and R2 upload.
• The step runs an inline Node.js script which checks release_files/latest.yml, release_files/latest-mac.yml, and release_files/latest-linux.yml, then writes release_files/health.json containing ok, version, per-platform ok flags and generated_at.
• The step reads the release version from RELEASE_VERSION (wired to needs.prepare.outputs.version) and fails the workflow with a clear error if any manifest is missing or empty.
• No changes required to the upload steps because files: release_files/* (GitHub) and the existing aws s3 sync release_files/ ... (Cloudflare R2) will include health.json automatically.

Testing

• Ran ruby -e "require 'yaml'; YAML.load_file('.github/workflows/build.yml'); puts 'YAML OK'" and it returned YAML OK indicating the workflow YAML is valid.
• Ran git diff --check and there were no check failures.
• Performed a local Node.js smoke test by creating a temporary release_files with latest.yml, latest-mac.yml, and latest-linux.yml, running the inline script with RELEASE_VERSION=1.2.3, and confirmed release_files/health.json was produced with top-level ok: true and version: "1.2.3".

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