Initialize a worktree’s checkpoint state to base ref.
Called when sync is enabled at session start to set the initial checkpoint
state. This indicates the worktree starts from a clean base ref state,
which is considered “synced” because the base ref bundle has already been
uploaded via workspace sync --enable.
This allows migration/resume from the base ref checkpoint.
Bearer authentication header of the form Bearer <token>, where <token> is your auth token.