chore(mobile): remove cloudSync v1 engine

Mirror of PR #052b (web) for mobile. Drops the dead-code v1 engine
tree under apps/mobile/src/sync/ now that ADR-0047 T₀ is past
(client cut-over completed 2026-05-06; v1 channel returns 410 Gone
since module_data table was dropped in commit 75dcdd5).

Deleted (23 files, ~2,597 LOC):
- engine/    (buildPayload, pull, push, replay, retryAsync)
- queue/     (collectQueued, deadLetter, offlineQueue)
- state/     (dirtyModules, moduleData, versions)
- net/       (online)
- helpers    (api.ts, config.ts, errorNormalizer.ts, events.ts,
              hook/useSyncCallbacks.ts)
- __tests__/ (deadLetter, offlineQueue, online, replay,
              useSyncedStorage)

Rewrote as stubs (5 files):
- enqueue.ts            no-op (kept for 17+ module-store call-sites
                        until PR #53 KVStore deprecate)
- hook/useSyncStatus.ts read-only stub returning idle shape
- types.ts              kept only SyncState/SyncError/CurrentUser
- useSyncedStorage.ts   useLocalStorage + enqueueChange (no-op) wrapper
- index.ts              barrel updated to remove dead-code exports

Refactored:
- persister/mmkvPersister.ts — QUERY_CACHE_KEY now imported from
  @sergeant/shared (STORAGE_KEYS.MOBILE_QUERY_CACHE) instead of
  deleted local config.ts.

Verification:
- pnpm lint                                   ✓
- pnpm typecheck                              ✓
- pnpm dead-code:files                        ✓ (no new orphans)
- mobile test (targeted on changed surface)   ✓ (no new failures;
  31 pre-existing failures share root cause: brandColors.cream
  undefined in src/theme.ts:43, unrelated to cloudSync)
- grep cloudSync|enqueueChange|getDirtyModules|getOfflineQueue
  apps/mobile/src                             ✓ zero hits in deleted
                                                files; only stub
                                                surface remains

Playbook: cleanup-dead-code.md
Roadmap: docs/planning/storage-roadmap.md §3 PR #052c
ADR:     docs/adr/0047-cloudsync-v1-410-gone.md (Phase 5 cut-over)

Author: cognition-team