refactor(config): replace senior-developer with role-specific specialists

- T2 frontend: software-architect → frontend-architect
- T3 backend: senior-developer → backend-developer
- T3 frontend: senior-developer → frontend-architect
- T3 default: senior-developer → backend-developer
- T4 backend: backend-architect → backend-developer (implementation, not architecture)
- T4 default: senior-developer → backend-developer

Depends on: coding-with-hans-heinemann/agency-agents#1 (new agent roles)

config/role_registry.yaml

@@ -3,7 +3,7 @@ t1:
 
 t2:
   backend:  agents/engineering/engineering-software-architect.md
-  frontend: agents/engineering/engineering-software-architect.md
+  frontend: agents/engineering/engineering-frontend-architect.md
   infra:    agents/engineering/engineering-devops-automator.md
   data:     agents/engineering/engineering-data-engineer.md
   ai:       agents/engineering/engineering-software-architect.md
@@ -12,8 +12,8 @@ t2:
   default:  agents/engineering/engineering-software-architect.md
 
 t3:
-  backend:  agents/engineering/engineering-senior-developer.md
-  frontend: agents/engineering/engineering-senior-developer.md
+  backend:  agents/engineering/engineering-backend-developer.md
+  frontend: agents/engineering/engineering-frontend-architect.md
   infra:    agents/engineering/engineering-sre.md
   data:     agents/engineering/engineering-data-engineer.md
   ai:       agents/engineering/engineering-ai-engineer.md
@@ -22,11 +22,11 @@ t3:
   database: agents/engineering/engineering-database-optimizer.md
   devops:   agents/engineering/engineering-sre.md
   docs:     agents/engineering/engineering-technical-writer.md
-  default:  agents/engineering/engineering-senior-developer.md
+  default:  agents/engineering/engineering-backend-developer.md
 
 t4:
   frontend:  agents/engineering/engineering-frontend-developer.md
-  backend:   agents/engineering/engineering-backend-architect.md
+  backend:   agents/engineering/engineering-backend-developer.md
   database:  agents/engineering/engineering-database-optimizer.md
   devops:    agents/engineering/engineering-devops-automator.md
   mobile:    agents/engineering/engineering-mobile-app-builder.md
@@ -35,7 +35,7 @@ t4:
   docs:      agents/engineering/engineering-technical-writer.md
   data:      agents/engineering/engineering-data-engineer.md
   embedded:  agents/engineering/engineering-embedded-firmware-engineer.md
-  default:   agents/engineering/engineering-senior-developer.md
+  default:   agents/engineering/engineering-backend-developer.md
 
 t5:
   code:          agents/engineering/engineering-code-reviewer.md