fix: 2026-06-04 audit pass — README, USB pack, multi-area fixes

WS ticket dashboard auth, builder universal signing/size limits/fusion obfuscation/dropper bundles, Path Tracer WireGuard topology, SessionGate degraded mode and download timeouts, server bootstrap (data dir, cloudflared dedupe, config port precedence), agent mesh/miner/spread fixes. README refreshed; usb bundle repacked; PROBLEMS.md audit log updated.

server/internal/api/handlers.go

@@ -58,6 +58,10 @@ func (h *Handler) GetAgent(w http.ResponseWriter, r *http.Request) {
 // GET /api/v1/agents/{id}/stats
 func (h *Handler) GetAgentStats(w http.ResponseWriter, r *http.Request) {
 	id := chi.URLParam(r, "id")
+	if _, err := h.db.GetAgent(id); err != nil {
+		http.Error(w, "Agent not found", http.StatusNotFound)
+		return
+	}
 	limitStr := r.URL.Query().Get("limit")
 	limit := 100
 	if l, err := strconv.Atoi(limitStr); err == nil && l > 0 {