fix(02-03): surface reverse-proxy record failures in 502 bodies

Silent "upstream error" hid capture-rule failures during MCP token exchange.

Co-authored-by: Cursor <cursoragent@cursor.com>

tide/proxy.go

@@ -107,7 +107,7 @@ func NewProxy(cfg ProxyConfig) (*Proxy, error) {
 		},
 		ModifyResponse: p.modifyResponse,
 		ErrorHandler: func(w http.ResponseWriter, r *http.Request, err error) {
-			http.Error(w, "tide: upstream error", http.StatusBadGateway)
+			http.Error(w, "tide: upstream error: "+err.Error(), http.StatusBadGateway)
 		},
 	}
 	return p, nil