DSH: close the fourth-Agent identity across all boundaries

## 目标

将 `dsh` 作为第四个 AgentKind 闭合到 core、Desktop、device-link、Mobile、DB 与远程边界，但不因类型存在而启用运行时。

## 交付

- 全仓 closed union/schema/serializer/decoder/selector inventory 与防遗漏测试。
- `dsh` 在 DB、IPC、URL、Mobile payload、搜索、scheduler、remote 与 UI 中保持身份。
- unknown/invalid 值 fail closed，不能降级为 Claude Code、Codex 或 Pi。

## 验收

- 保持现有三种 Agent 的持久化、创建、切换、搜索、scheduler 和 Mobile 回归。
- availability 仍由后续受管 runtime / Cindy bridge handshake 决定。

Blocked by #3771. Blocks #3773.

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

Start with docs/issues/dsh-native-integration/ and the prerequisite work in #3771; the issue identifies the core, Desktop, device-link, Mobile, DB, remote, IPC, URL, scheduler, search, and UI boundaries to inventory. Done means dsh remains distinct across persistence and interactions, invalid values fail closed, existing three-Agent regressions pass, and availability is not enabled until the later runtime handshake; coordinate with the blocking issue before starting.

Written by the indexing model from the issue text.