chore: repoint vendor/zoekt at sourcebot-dev/zoekt@main merge commit

sourcebot-dev/zoekt#13 merged as 7c6c629f. Updating the submodule
pointer from the feature-branch tip (945c3e96) to the merge commit
on main so vendor/zoekt tracks canonical history before merging.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: msukkari