Automations don't work with Jujutsu VCS changes

### Short summary

Automations fail to run because my repo uses JJ for version control which puts the underlying Git repo in a Detached HEAD state

### Affected version or release

v1.0.6

### Installation context

_No response_

### What happened?

When automations get triggered, the following error message is surfaced:
`Branch workflow cannot start from a detached HEAD. Check out the branch you want this workflow to run on, then run it again.`

### Steps to reproduce

1. Initialize JJ in a Git repo
2. Create a new JJ change
3. Create an automation on that repo

### Expected behavior

The automation is able to run successfully

### Additional context

_No response_

Reproduce the issue using the listed steps: initialize Jujutsu in a Git repository, create a JJ change, and trigger an automation. Start from the detached-HEAD error path and verify how automation startup handles JJ-managed repositories. Done means the automation runs successfully instead of reporting that the workflow cannot start from a detached HEAD.

Rédigé par le modèle d'indexation à partir du texte de l'issue.