Relative worktrees not supported

**Description**

When starting `gitui` on a repository with relative worktrees the following error happens:

```
Error: invalid repo path: unsupported extension name extensions.relativeworktrees; class=Repository (6)
```

**Steps to reproduce**
```
mkdir test-repo && cd test-repo
git init
git commit --allow-empty -m "initial commit"
git worktree add --relative-paths -b dev-branch ../test-repo.worktrees/dev-branch
gitui
```

**Expected behavior**

`gitui` starts without error.

**Context:**

- `macOS 26.4.1`
- `gitui 0.28.1-nightly 2026-03-24`
- `git version 2.50.0`

Reproduce the issue with the provided git init and git worktree commands, then start at gitui's repository-opening and worktree/path-handling entry points. Trace how the relative worktree configuration is read and run the existing test suite; done means gitui starts successfully on the reproduced repository without the unsupported extension error.

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