[Epic]: Simplify and standardize issue templates

### User Story

As a project team, I want a simple, consistent set of GitHub issue templates, so that contributors can create appropriately scoped and actionable work items.

### Description

Simplify and standardize the repository’s GitHub issue templates to support a clear work-item hierarchy and a low-friction issue-creation experience.

This Epic will define the intended hierarchy and template purposes; audit the current templates; propose and approve a streamlined template set; implement the approved configuration; and validate the resulting author experience.

Scope:
- Agile work-item templates: Initiative, Feature, Epic, Story, Task, and Defect.
- Keep onboarding and offboarding templates as separate operational checklists, while reviewing them only for consistency and maintenance needs.

Out of scope:
- Reorganizing existing open work items. That work follows after the hierarchy and templates are approved.

### Success Metrics

- The approved work-item hierarchy and purpose of each type are documented.
- The approved template set eliminates known naming and field mismatches.
- Every active template is concise, usable, and aligned to its work-item type.
- Issue authors can create each work-item type with the information required for triage and delivery.

### Additional Information

Dependencies: agreement from project leadership/product on hierarchy definitions and any GitHub Projects configuration needed to support it.

Child work items will cover hierarchy definition, template audit, proposed design, implementation, and end-to-end validation.