feat: expand role_registry with specialist roles + update agency-agents submodule (#4)

Role registry changes:
- T2 backend: software-architect → backend-architect
- T2 frontend: software-architect → frontend-architect
- T3 backend: senior-developer → senior-backend-developer (NEW)
- T3 frontend: senior-developer → senior-frontend-developer (NEW)
- T4 backend: backend-architect → backend-developer
- T4 default: senior-developer → backend-developer
- Added coverage for: ai, security, mobile, database, devops, docs, data, embedded, e2e, accessibility

Submodule updated to include: frontend-architect, backend-developer,
senior-backend-developer, senior-frontend-developer.

Clean tier separation:
  T2 = architects (design)
  T3 = senior devs (lead + implement-or-delegate)
  T4 = developers (pure implementation)
  T5 = reviewers/testers (verification)

config/role_registry.yaml

@@ -2,33 +2,49 @@ t1:
   default: agents/strategy/nexus-strategy.md
 
 t2:
-  backend:  agents/engineering/engineering-software-architect.md
-  frontend: agents/engineering/engineering-software-architect.md
+  backend:  agents/engineering/engineering-backend-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
+  security: agents/engineering/engineering-security-engineer.md
+  mobile:   agents/engineering/engineering-software-architect.md
   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-senior-backend-developer.md
+  frontend: agents/engineering/engineering-senior-frontend-developer.md
   infra:    agents/engineering/engineering-sre.md
-  default:  agents/engineering/engineering-senior-developer.md
+  data:     agents/engineering/engineering-data-engineer.md
+  ai:       agents/engineering/engineering-ai-engineer.md
+  security: agents/engineering/engineering-security-engineer.md
+  mobile:   agents/engineering/engineering-mobile-app-builder.md
+  database: agents/engineering/engineering-database-optimizer.md
+  devops:   agents/engineering/engineering-sre.md
+  docs:     agents/engineering/engineering-technical-writer.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
   ai:        agents/engineering/engineering-ai-engineer.md
   security:  agents/engineering/engineering-security-engineer.md
   docs:      agents/engineering/engineering-technical-writer.md
-  default:   agents/engineering/engineering-senior-developer.md
+  data:      agents/engineering/engineering-data-engineer.md
+  embedded:  agents/engineering/engineering-embedded-firmware-engineer.md
+  default:   agents/engineering/engineering-backend-developer.md
 
 t5:
-  code:        agents/engineering/engineering-code-reviewer.md
-  integration: agents/testing/testing-reality-checker.md
-  api:         agents/testing/testing-api-tester.md
-  performance: agents/testing/testing-performance-benchmarker.md
-  security:    agents/engineering/engineering-security-engineer.md
-  default:     agents/engineering/engineering-code-reviewer.md
+  code:          agents/engineering/engineering-code-reviewer.md
+  integration:   agents/testing/testing-reality-checker.md
+  api:           agents/testing/testing-api-tester.md
+  performance:   agents/testing/testing-performance-benchmarker.md
+  security:      agents/engineering/engineering-security-engineer.md
+  accessibility: agents/testing/testing-accessibility-auditor.md
+  e2e:           agents/testing/testing-evidence-collector.md
+  frontend:      agents/testing/testing-accessibility-auditor.md
+  data:          agents/testing/testing-reality-checker.md
+  default:       agents/engineering/engineering-code-reviewer.md