Set up OVSX_PAT secret for Open VSX publishing

[mkcorneli]

The release workflow (PR #28) supports publishing to Open VSX, but the OVSX_PAT repository secret hasn't been configured yet. The workflow skips this step gracefully when the secret is missing.

To do:

1. Create an access token at https://open-vsx.org/user-settings/tokens
2. Add it as a repository secret named OVSX_PAT (Settings → Secrets → Actions)

Once set, future releases will auto-publish to Open VSX alongside VS Marketplace.

[mikhail-zhadanov]

done