Allow templates for design doc and implementation plans

SpecKit lets you customize the templates you use for the design doc and implmentation plan, we should make these editable per-project as well:

```
.specify/templates/
  ├───agent-file-template.md
  ├───checklist-template.md
  ├───plan-template.md
  ├───spec-template.md
  └───tasks-template.md
```