/simplify: make non-branch ref push non-force too (one consistent policy)

Altitude follow-up to the non-force checkpoint push: tryPushRefCommon (the v1-era per-ref pusher behind doPushRef) still force-pushed non-branch refs (+ref:ref), which contradicts the new fast-forward-only policy for per-checkpoint refs and was a latent footgun — a future caller routing a checkpoint ref through doPushRef would silently overwrite a divergence. The non-branch path has no production caller today (the v1 PrePush loop only pushes the v1 branch), so dropping the force is safe and unifies the policy: every checkpoint ref, branch or per-checkpoint, is fast-forward-only with doPushRef's fetch+rebase recovery on divergence.

Also clarify (comment) that explain_export's RefName-error branch is defensive — cid is already validated by NewCheckpointID, so it can't fire — rather than a swallowed live error.