CI: Publish OTA release on merged PRs to main and remove artifact upload

[shai-almog]

Motivation

• Replace per-job artifact uploads with a GitHub Release flow so full OTA content can be published automatically when a PR is merged to main.

Description

• Changed workflow trigger to pull_request types: [closed] targeting main and gated the job with if: github.event.pull_request.merged == true so the job runs only for merged PRs.
• Removed the actions/upload-artifact step and added a packaging step that archives all non-hidden files under OTA/ into dist/ota-${GITHUB_SHA}.tar.gz using find and tar (excluding dotfiles and hidden directories).
• Added a release publishing step using softprops/action-gh-release@v2 which publishes the created tarball as a release asset and uses tag_name: ota-${{ github.sha }} and name: OTA Release ${{ github.sha }}.
• Set workflow permission contents: write and updated checkout action to actions/checkout@v4.

Testing

• Ran ./build_skins.sh locally; the script completed (exit code 0) but emitted expected warnings about missing theme files and some zip I/O messages in this checkout.
• Verified the updated workflow file at .github/workflows/blank.yml and committed the change successfully.

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Codex Task