chore: remove ArcheHelix branding, use plain PDCA language

The archetypes and shadows are distinctive enough — no need for
a fancy name on top of the standard PDCA cycle terminology.

examples/custom-workflow.yaml

@@ -4,8 +4,8 @@
 name: api-design
 description: "API-first workflow with contract validation and adversarial testing"
 
-# The ArcheHelix configuration
-archehelix:
+# PDCA workflow configuration
+pdca:
   plan:
     archetypes: [explorer, creator]
     parallel: false  # sequential: Explorer feeds Creator