Export/import automations as version-controlled JSON (config-as-code for Automations)

### Summary
Requesting a way to export the Automations list to a plain JSON file (and import from one), so automations can be version-controlled, backed up, and shared/restored across machines - similar to how other app config (MCP servers, permissions, settings) is already stored as flat JSON under `~/.copilot/`.

### Motivation
Automations currently live only inside the app's internal SQLite store (`data.db`), with no export path. This makes it hard to:
- Back up or restore automations independent of full app state
- Version-control automation definitions alongside a repo/dotfiles setup
- Migrate automations to a new machine

Issue #1643 shows what happens without this: a user hit a `data.db` schema-version mismatch during a downgrade and had no way to recover automations short of deleting the whole database and losing session history with it.

### Proposed solution
- "Export automations to JSON" / "Import automations from JSON" actions in the Automations UI, or
- A flat, human-editable automations file (mirroring how `mcp-config.json`/`settings.json`/`permissions-config.json` already work) that the app reads on startup, so power users can hand-edit and symlink it into version control

### Additional context
Related to the scheduling limitations raised in #2130 - a config-as-code path would also make it easier to share/reuse automation definitions across teams or repos.

首先比较 ~/.copilot/ 下现有的扁平 JSON 配置文件——包括 mcp-config.json、settings.json 和 permissions-config.json——与 data.db 中的自动化存储。确定 Automations UI 中的导出/导入操作还是启动时加载的文件才是预期路径，然后验证自动化定义是否可以在不影响会话历史的情况下进行备份、恢复、编辑并在不同机器之间迁移。

由索引模型根据 Issue 内容生成。