DSH: make device-link and Mobile task continuity explicit

## 目标

让 Mobile 经受控 Desktop 连续查看和控制同一 DSH task，且在版本差异和断线时诚实降级。

## 交付

- append-only identity/capability/activity/approval/cancel payload、Desktop allowlist、Mobile reducer/UI。
- view/continue/input/attachment/approval/stop/queue 与 read-only activity capability gates。

## 验收

- Mobile 不直连 ACP transport、scope 或 credential，不创建 substitute session。
- old/new Desktop × Mobile matrix 与 two-peer failure radius 通过。

Blocked by #3779. Blocks #3781.

Canonical plan: `docs/issues/dsh-native-integration/`.

Read docs/issues/dsh-native-integration/ and resolve dependency #3779 first. Then trace the Desktop allowlist, Mobile reducer/UI, append-only payloads, and capability gates named in the issue. Use the old/new Desktop × Mobile matrix and two-peer failure-radius checks as validation; done means the listed transport, scope, credential, and substitute-session constraints hold.

Written by the indexing model from the issue text.