ossutil 2.x signs requests with V4 and refuses to run without an
explicit region, so the OSS mirror sync would fail in CI even with
valid credentials. Derive OSS_REGION from the endpoint host
(including -internal variants) and fail fast when it cannot be
derived.
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.