Hint at --force-blind on receive-pack lease failures

When a non-BestEffort push gets a CommandStatusErr whose status looks like a lease miss (stale info / fetch first / non-fast-forward / does not match expected old value), wrap it with "target ref X moved or differs from session start; rerun, or use --force-blind to overwrite."

The wrapper preserves the underlying CommandStatusErr via fmt.Errorf %w, so callers and tests that inspect the typed error keep working; the hint is purely additive on the error message.

Closes the reason-text portion of #47.