Replace C4-level nested labels by flat labels

Nested labels at the C4 level (e.g., project::) cannot be completed or extended by the squad. For example, if the squad creates a label like project::bluecoataway, GitLab treats it as a new, separate label rather than a sublabel of project::.

To ensure consistent labeling and ease of use, we propose replacing nested labels with simple flat labels that squads can use directly and then create separate ones.