feat: phase 3-5 — workflow engine, background subagents, multi-modal, cache shape, inline diff

Phase 3: Dynamic Workflow Engine
- VM sandbox (node:vm) with agent/parallel/pipeline API, Claude Code compatible
- Workflow file discovery (.boocode/workflows/*.js + ~/.boocode/workflows/*.js)
- Workflow manager with session/chat creation and inference dispatch
- Built-in catalog: deep-research, review-code, find-issues
- Resumability cache: SHA-256 hash of agent spec, in-memory Map

Phase 4: Background Subagents
- background-task.ts service: spawn/poll/cancel lifecycle
- spawn_subagent, subagent_status, subagent_result tools in ALL_TOOLS

Phase 5: Multi-modal + Cache Shape
- Multi-modal stub with type defs and hook point in payload.ts
- CacheShapeBadge component in trace viewer (colored bar + %)

apps/server/src/services/inference/multi-modal.ts

@@ -0,0 +1,56 @@
+// vDeepSeek (stub): multi-modal (image) attachment support.
+//
+// When a message carries images, DeepSeek V4 models can process them
+// natively via the @ai-sdk/deepseek provider. This module provides the
+// helper types and functions to detect and convert image attachments.
+//
+// FULL INTEGRATION requires:
+//   1. Storing image data alongside messages (message_parts with kind='image'
+//      or a dedicated attachments table with base64-encoded data).
+//   2. Extending OpenAiMessage.content from `string | null` to
+//      `string | null | Array<{ type: 'text'; text: string } | { type: 'image'; image: string }>`
+//      in apps/server/src/services/inference/payload.ts.
+//   3. Updating toModelMessages() in stream-phase-adapter.ts to emit AI SDK
+//      content arrays with image parts for multimodal user messages.
+//
+// None of the above is done yet — this file is a type scaffold.
+
+import type { Message } from '../../types/api.js';
+
+/** Shape of a decoded image attachment ready for the AI SDK. */
+export interface ImageAttachment {
+  /** Base64-encoded image data (no data URI prefix — raw bytes). */
+  data: string;
+  /** MIME type (e.g. 'image/png', 'image/jpeg', 'image/webp'). */
+  mimeType: string;
+}
+
+/**
+ * Check if a user message has image content that can be forwarded to a
+ * multimodal model. Currently a stub — always returns false until the
+ * message-pipeline stores image attachments addressably.
+ */
+export function hasImageAttachments(_message: Message): boolean {
+  // TODO(vDeepSeek): scan message_parts for kind='image' or inspect
+  // message.content for inline data URIs (data:image/...).
+  return false;
+}
+
+/**
+ * Convert internal image attachments to the format expected by the AI SDK
+ * ModelMessage content array.
+ *
+ * The @ai-sdk/deepseek provider accepts images as:
+ *   { type: 'image'; image: 'data:image/png;base64,...' }
+ *
+ * @param attachments — List of decoded image attachments.
+ * @returns AI SDK inline file parts suitable for ModelMessage.content.
+ */
+export function imageAttachmentsToParts(
+  attachments: ImageAttachment[],
+): Array<{ type: 'image'; image: string }> {
+  return attachments.map((a) => ({
+    type: 'image' as const,
+    image: `data:${a.mimeType};base64,${a.data}`,
+  }));
+}