feat: add PR validation workflow and contribution checks

bmad-core/workflows/greenfield-ui.yaml

@@ -64,7 +64,7 @@ workflow:
       condition: po_checklist_issues
       notes: "If PO finds issues, return to relevant agent to fix and re-export updated documents to docs/ folder."
 
-    - project_setup_guidance:
+    - step: project_setup_guidance
       action: guide_project_structure
       condition: user_has_generated_ui
       notes: "If user generated UI with v0/Lovable: For polyrepo setup, place downloaded project in separate frontend repo. For monorepo, place in apps/web or frontend/ directory. Review architecture document for specific guidance."
@@ -133,7 +133,7 @@ workflow:
         - Dev Agent (New Chat): Address remaining items
         - Return to QA for final approval
 
-    - repeat_development_cycle:
+    - step: repeat_development_cycle
       action: continue_for_all_stories
       notes: |
         Repeat story cycle (SM → Dev → QA) for all epic stories
@@ -150,7 +150,7 @@ workflow:
         - Validate epic was completed correctly
         - Document learnings and improvements
 
-    - workflow_end:
+    - step: workflow_end
       action: project_complete
       notes: |
         All stories implemented and reviewed!