Use directory structure that supports multiple features

[johnpryan]

We should make it easier to create design docs and implementation plans for multiple features by placing files into a folder.

For example, SpecKit uses this type of folder structure for organizing specifications for multiple features within the same project:

specs/
  ├───001-okr-management-app/
  │   ├───checklists/
  │   │   └───requirements.md
  │   ├───data-model.md
  │   ├───plan.md
  │   ├───quickstart.md
  │   ├───research.md
  │   ├───spec.md
  │   └───tasks.md
  └───002-i-need-the/
      ├───checklists/
      │   └───requirements.md
      ├───plan.md
      ├───quickstart.md
      ├───research.md
      └───spec.md