feat(canvas): attach uploaded image names to model prompts

Add modelImageReferences to preprocess agent contents, uploading local
image references and surfacing their names in the prompt so the model can
cite them. Wire the preparer into the canvas agent submission path, pass
imageName through the floating composer references, and relax the prompt
directive parser to accept names containing arbitrary characters.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/ui/pages/CanvasWorkspace/index.tsx

@@ -39,6 +39,7 @@ import { SharePopover } from "@/ui/components/SharePopover";
 import { autoImageAdjustments, defaultImageAdjustments, parseImageAdjustments, serializeImageAdjustments } from "@/ui/lib/imageAdjustments";
 import { friendlyProjectFailureMessage } from "@/ui/lib/friendlyAgentErrors";
 import { canvasImageUrl } from "@/ui/lib/imageDelivery";
+import { createModelAgentContentPreparer } from "@/ui/lib/modelImageReferences";
 import { visiblePromptText } from "@/ui/lib/promptImageReferences";
 import { isImageGeneratorThreadMessage, visibleAgentMessages } from "./canvas/agentMessageFilters";
 import { CanvasColorPopover } from "./canvas/CanvasColorPopover";
@@ -90,6 +91,7 @@ const defaultCanvasBackgroundColor = "#F7F7F4";
 const canvasBackgroundSwatches = ["#F5F5F5", "#000000", "#FFFFFF", "#00F016", "#A855F7", "#E9D5FF"];
 const canvasSidePanelWidth = 268;
 const newAgentConversationId = "__new_agent_conversation__";
+const prepareModelAgentContents = createModelAgentContentPreparer({ uploadImage: designGateway.uploadImage });
 const canvasWheelGuardSelector = [
   ".ui-popover-content",
   ".ui-dropdown-menu-content",
@@ -2769,10 +2771,11 @@ export function CanvasWorkspace({ projectId, shareAccess = ownerWorkspaceAccess
       if (dirty) {
         await saveCanvasSnapshot({ quiet: true });
       }
+      const modelContents = await prepareModelAgentContents(contents);
       startAgentSocket(
         project.id,
         {
-          messages: [{ role: "user", contents }],
+          messages: [{ role: "user", contents: modelContents }],
           mode: source === "image-generator" ? "image-generator" : activeMode === "image" ? "image" : "design",
           threadId,
           threadIdType: 9,