Snap strict confinement: GitUI misreports repository NotFound as “non-bare repository”

**Describe the bug**
Using the snap version of gitui in strict confinement mode, running `gitui` in a valid non-bare repository outside snap-allowed paths (for example `/i4`) shows a misleading error:

invalid path
please run gitui inside of a non-bare git repository

The repository is actually non-bare. The real issue is repository discovery failure (`NotFound`), not a bare-repository problem.

**To Reproduce**
Steps to reproduce the behavior:
1. Install gitui from snap:
snap install gitui
2. Create a repository outside `/home` (for example `/i4/repo`):
mkdir -p /i4/repo
cd /i4/repo
git init
3. Verify the repository is non-bare:
git rev-parse --is-bare-repository
(output should be `false`)
4. Run `gitui` and see error.

**Expected behavior**
gitui should report the actual cause (repository open/discovery failure due to snap strict path confinement), instead of incorrectly telling users to run inside a non-bare repository.

**Screenshots**
N/A. Terminal output is included above.

**Context (please complete the following information):**
- OS/Distro + Version: Ubuntu 24.04.3 LTS
- GitUI Version [e.g. `0.5`]: snap package (strict confinement)
- Rust version: N/A (prebuilt snap binary)

**Additional context**