Define work-item hierarchy and template conventions

### Description

Document the purpose, scope, and parent/child relationship for Initiatives, Features, Epics, Stories, Tasks, and Defects. Establish a shared vocabulary that will drive template design and later work-item reorganization.

### Action Items

- [ ] Facilitate a decision on the target hierarchy.
- [ ] Define when each work-item type should be used.
- [ ] Document required relationships and minimum fields for each type.
- [ ] Obtain approval from project leadership/product.

### Additional Information

Parent Epic: #726

Start with the parent Epic, #726, then review this issue’s action items as the scope for the work. Define and document the target hierarchy, usage guidance, relationships, and minimum fields for Initiatives, Features, Epics, Stories, Tasks, and Defects. The work is done when project leadership/product approves the conventions.

Written by the indexing model from the issue text.