[Feature] Enable to `git add` reviewed files changed

I would love to have a "Git" integrated feature, where when reviewing "Files changed", I could stage reviewed files (as if they'd been `git add`-ed).

---
| Field | Value |
| --- | --- |
| App version | 1.1.8 |
| OS | macOS 26.5.0 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Week 8 |

Start by tracing the Files changed review flow from /chat and locating the app entry point for Git-related actions. Determine how reviewed files are represented and how staging should be exposed; done means reviewed files can be staged as though git add was run, with the behavior verified in the app.

索引モデルが issue の本文から書いたものです。