[Requests and ideas] Allow opening subfolder in large monorepo

**Mood:** 😊
**Category:** Requests and ideas

We’re using a large monorepo containing multiple sub-applications, where each application has its own .github folder with Copilot instructions and skills.

The current challenge is that GitHub Copilot App always opens the repository from the repository root. Because of that, instructions and skills located inside sub-application folders are not automatically discovered or applied in a clean way.

Example:

```
repo/
.github/
app1/
.github/
instructions/
skills/
app2/
.github/
instructions/
skills/
```

In practice, this makes it difficult to have isolated Copilot context per application inside a monorepo.

What we would like is either:

- The ability to open a specific subdirectory/application as the effective workspace root
- Automatic discovery and inheritance of .github instructions/skills from the nearest parent directory of the files being worked on
- Better first-class monorepo support for application-scoped Copilot configuration

Right now, monorepo setups require centralizing everything into the repository root, which becomes difficult to maintain for large repositories with multiple independent applications and teams.

---
| Field | Value |
| --- | --- |
| App version | 0.2.8 |
| OS | Windows 10.0.26100 |
| Theme | Fox |
| Path | /chat |
| Tenure | Day 2 (Week 1) |

从 /chat 路径上的仓库根目录打开行为入手，然后检查如何在 repo/、app1/ 和 app2/ 中发现 .github 指令和 skills。完成的标准是：选定的子目录可以作为有效的工作区根目录，并且应用范围的配置能够正常工作，而无需将所有内容集中到仓库根目录。

由索引模型根据 Issue 内容生成。