Add remote proxy lease backend for HarmonyOS (harmonyos-instance)

## Problem
The 0.20.10 proxy inventory lists booted HarmonyOS devices, and the local host provider supports `open --platform harmonyos`, but remote proxy lease admission rejects HarmonyOS because `LeaseBackend` only contains `ios-simulator`, `ios-instance`, and `android-instance`.

Observed from a Linux client connected to a Mac proxy:
- `devices --platform harmonyos` lists a booted `emulator`.
- `open ... --platform harmonyos --device emulator` fails before allocation with `open requires --platform ios|android or --lease-backend`.
- The local Mac (non-proxy) can open the same Harmony emulator and exposes snapshot/click/type/logs/screenshot.

## Proposal
Add a first-class `harmonyos-instance` lease backend end-to-end: contracts/type, platform-to-backend mapping, proxy lease allocate/heartbeat/close, session runtime hints, and tests proving device selection and lease lifecycle. Preserve fail-closed behavior for unsupported providers; do not merely widen a client enum.

Acceptance should include remote proxy inventory -> open -> snapshot/input/logs -> close, with lease identity and artifact provenance. Physical OHOS remains out of scope; emulator/proxy only.