Sidebar: sessions for the same working directory keep moving between two groups (folder name vs. git remote name)

## Environment

- Claude Code desktop app, **Code** tab
- Version: 2.1.234
- OS: Windows 11 Pro, 10.0.26200

## Summary

In the desktop app's session sidebar, the group heading for one and the same working directory is sometimes derived from the **folder name** and sometimes from the **git remote name**. Both variants can exist at the same time, so sessions that share a working directory are split across two groups. The grouping also changes on its own between app sessions, and sessions visibly move from one group to the other without any user action.

In my case the folder name and the repository name differ, e.g.:

- Working directory: `C:\Users\\OneDrive - \Documents\...\My Project Folder`
- Git remote: `github.com//my-repo`

The sidebar shows both `My Project Folder` and `my-repo` as separate groups, and sessions migrate between them.

## Steps to reproduce

1. Have a local git repository whose folder name differs from the repository name in the git remote.
2. Start several sessions in that directory (some directly in the repo root, some in git worktrees created under `.claude/worktrees/`).
3. Look at the sidebar; restart or reload the app a few times.

## Expected behavior

Sessions with the same working directory (and worktrees of the same repository) stay in one stable group with a stable name.

## Actual behavior

- Two groups exist for the same repository: one named after the folder, one named after the git remote.
- Sessions move between the two groups over time.
- Which sessions land in which group is not predictable — worktree sessions and repo-root sessions have been split at times and merged at other times.

## Additional notes

- Making the grouping deterministic (always one key per repository, or always the working directory) would be enough.
- A visible indicator of the group key would also help, since it is currently impossible to tell why a session was filed where.

Reproduce this in the desktop app's Code tab on Windows using a repository whose folder and git remote names differ, including repo-root and .claude/worktrees/ sessions, then restart or reload repeatedly. Trace how the sidebar chooses group names and verify that sessions for the same repository or working directory remain in one stable group with a deterministic name.

索引モデルが issue の本文から書いたものです。