feat: add supervisor prototype with embedded frontend

internal/supervisor/manager.go

@@ -0,0 +1,13 @@
+package supervisor
+
+import "sync"
+
+// Manager is a future extension point for high-level orchestration.
+type Manager struct {
+	mu          sync.RWMutex
+	assignments map[string]AgentAssignment
+}
+
+func NewManager() *Manager {
+	return &Manager{assignments: make(map[string]AgentAssignment)}
+}