Experimental worktree support doesn't work when workspaceMount is customized

I'm using Dev Containers with the experimental git worktree support setting enabled. My setup is a git worktree that's a sibling to the main repository:

Issue: When my devcontainer.json includes a custom workspaceMount configuration, the experimental worktree feature does not mount the main repository's .git directory. The worktree's .git file references a relative path (e.g., gitdir: ../main-repo/.git/worktrees/my-worktree) that doesn't resolve inside the container.

Reproduction steps:

1. Create a git worktree as a sibling to the main repository
2. Have a devcontainer.json inside the worktree with a custom workspaceMount:

{
  "workspaceMount": "source=${localWorkspaceFolder}/../../,target=/workspace,type=bind",
  "workspaceFolder": "/workspace/services/my-service"
}

3. Enable experimental worktree support
4. Open the folder in a devcontainer
5. Git source control doesn't work — the path in the worktree's .git file doesn't exist
   Expected behavior: The experimental feature should detect the worktree and mount the main repository's .git directory even when workspaceMount is customized.

Workaround: Removing the custom workspaceMount configuration allows the experimental feature to work correctly. It appears the feature only auto-mounts the main .git when using the default mount behavior.

(@chrmarti )