fix: pin SDK to known-good 500-image build SHA

Pin the vendored software-agent-sdk submodule to 30819566b56b592cedcd1dc0d503a454f8ca6668, which is the last SDK commit verified by successful end-to-end 500-image SWE-Bench and SWT-Bench builds.

Co-authored-by: openhands <openhands@all-hands.dev>

Author: simonrosenberg