feat(speckit): integrate ux reference into workflows

Introduce a UX reference stage to ensure technical plans align with
user experience goals. Adds a new template, a generation step in the
specification workflow, and mandatory validation checks during
planning to prevent technical compromises from degrading the defined
user experience.

.specify/templates/spec-template.md

@@ -1,6 +1,7 @@
 # Feature Specification: [FEATURE NAME]
 
 **Feature Branch**: `[###-feature-name]`  
+**Reference UX**: `[ux_reference.md]` (See specific folder)
 **Created**: [DATE]  
 **Status**: Draft  
 **Input**: User description: "$ARGUMENTS"