fix: restore skills-vendor submodule to correct commit

The merge with upstream/master reset the submodule pointer to 308c4d4
which predates the reference files added in skills#25. Restore to
d0a72ea which includes dashboard-guide.md, dashboard-cards.md, and
domain-docs.md needed by the tests and this PR's functionality.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kingpanther13