[codex] Fix release artifact validation

[hackjutsu]

Summary

Fixes the release workflow after the v2.0.0-beta.1 prerelease run exposed a Windows packaging/artifact gap.

• Invokes the installed electron-builder CLI directly instead of going through npx in platform release jobs.
• Adds .github/scripts/assert-release-artifacts.js to print produced dist files and fail the build step when no top-level package artifact exists.
• Keeps upload globs limited to top-level release artifacts so unpacked app internals do not mask missing installers.

Validation

• ruby -e "require 'yaml'; YAML.load_file('.github/workflows/release.yml'); puts 'release workflow yaml ok'"
• node --check .github/scripts/assert-release-artifacts.js
• git diff --check
• node ./.github/scripts/assert-release-artifacts.js macOS .dmg .zip against stale local packaged output, to verify diagnostics/top-level artifact detection.

Release note

After this merges, use a new prerelease tag from the fixed master commit, preferably v2.0.0-beta.2, rather than reusing the failed v2.0.0-beta.1 tag.