feat: add typed Poimen REST endpoints and wire workflow types (#32, #34)

- New internal/temporal/poimen.go: PoimenHandler with typed REST endpoints
  POST /v1/poimen/plan — call PlannerActivity (stub until workflows#23)
  POST /v1/poimen/execute — start DAG/Orchestrator/Routing workflows
  GET  /v1/poimen/executions — list Poimen workflow executions
  GET  /v1/poimen/executions/:id — describe execution
  GET  /v1/poimen/executions/:id/history — activity event history
  GET  /v1/poimen/executions/:id/stream — SSE events (delegates to sse.go)
  POST /v1/poimen/executions/:id/signal — pause/resume/cancel
  DELETE /v1/poimen/executions/:id — terminate workflow
- New internal/serviceadapter/poimen_adapter.go: ServiceAdapter spec + factory
- Wire PoimenHandler in cmd/gateway/main.go via ServiceAdapter registry
- Workflow type registry: dag->DAGWorkflow, orchestrator->OrchestratorWorkflow,
  routing->RoutingWorkflow (all on poimen-worker task queue)

Closes #32
Closes #34

cmd/gateway/main.go

@@ -108,6 +108,10 @@ func main() {
 	}
 	_ = registry.Add(notifAdapter)
 
+	// Add Poimen orchestrator adapter (typed REST for AI orchestrator frontend)
+	poimenAdapter := serviceadapter.NewPoimenAdapter(temporalHandler)
+	_ = registry.Add(poimenAdapter)
+
 	// Add other adapters from config (skip if already registered in code)
 	for _, a := range cfg.Adapters {
 		if existing := registry.Get(a.ServiceName); existing != nil {