Open a session from a subfolder within a repo rather than root repo directory

I want to be able to open a session from a subfolder within a repo rather than only open it at the root directory of a repo

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| Field | Value |
| --- | --- |
| App version | 1.0.24 |
| OS | macOS 26.5.2 |
| Theme | Notionish |
| Path | /chat |
| Tenure | Week 4 |

The issue names no files, tests, or entry points. Start by tracing how the app selects the repository root when opening a session, then determine where a selected subfolder can be preserved. Done means opening a session from a repository subfolder works while existing root-directory behavior remains intact.

索引モデルが issue の本文から書いたものです。