Add scripts/release/sync-to-oss.sh and wire it into release.yml. After
the GitHub release, artifacts (binaries, checksums, dws-skills.zip) plus
the install scripts are pushed to an OSS bucket in the
<base>/<version>/<file> layout the install.sh DWS_RELEASE_BASE switch
expects, with a latest.txt pointer.
Gated on OSS_* repo secrets: skips cleanly when unset, so forks without
the secrets keep releasing to GitHub only.
