oven: add CL variant OTA build + channel-based upload
- native-builder: after C build succeeds, builds CL variant if
fedac/native/cl/ files changed (non-blocking, failure doesn't
fail the C build)
- upload-release.sh: OTA_CHANNEL env var prefixes upload paths
(e.g. OTA_CHANNEL=cl → cl-native-notepat-latest.vmlinuz)
- Separate releases.json per channel
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>