Run the git-refs canary in PR CI (matrix over checkpoint backend)

ci.yml's test-canary job now runs as a matrix over checkpoint_store [git-branch, git-refs], setting E2E_CHECKPOINT_STORE. The Vogon canary makes no API calls, so this guards the git-refs store on every PR at no cost alongside the default git-branch run. fail-fast: false so one backend's failure doesn't mask the other.

(The e2e-checkpoint-store dispatch workflow already landed separately on main in #1567; this branch leaves it untouched.)