mobile: thin hubClient re-export over shared SSOT (T80.2)

## Goal
Strangle mobile hubClient to thin shell: re-export shared types/methods; keep SecureStore refresh glue, fixture snapshot, legacy WS types only.

## Acceptance
- [ ] No new Hub REST methods added only on mobile
- [ ] LOC shrink vs 737 baseline
- [ ] `pnpm --filter agenthub-mobile-rn typecheck` green
- [ ] Existing mobile tests still pass if present

## Super
R,P

## Delivery Batch
B2