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/web/src/components/CacheShapeBadge.tsx

@@ -0,0 +1,38 @@
+// vDeepSeek: cache shape telemetry badge. Displays cache token count with
+// a colored hit-rate bar in the trace viewer. Color thresholds are relative
+// to output tokens (tokens_used) since the trace doesn't carry prompt miss
+// tokens separately: green > 50%, yellow > 10%, red ≤ 10%.
+
+export interface CacheShapeBadgeProps {
+  cacheTokens: number | null | undefined;
+  totalTokens: number | null | undefined;
+}
+
+function hitRate(cache: number, total: number): number {
+  if (cache <= 0 || total <= 0) return 0;
+  return cache / (cache + total);
+}
+
+function barColor(rate: number): string {
+  if (rate > 0.5) return 'bg-green-500';
+  if (rate > 0.1) return 'bg-yellow-500';
+  return 'bg-red-500';
+}
+
+export function CacheShapeBadge({ cacheTokens, totalTokens }: CacheShapeBadgeProps) {
+  if (cacheTokens == null || cacheTokens <= 0) return null;
+
+  const rate = hitRate(cacheTokens, totalTokens ?? 0);
+  const pct = Math.round(rate * 100);
+  const color = barColor(rate);
+
+  return (
+    <span className="shrink-0 inline-flex items-center gap-1 font-mono tabular-nums text-[10px] text-muted-foreground/60" title={`cache hit rate ${pct}%`}>
+      <span className={`inline-block w-1.5 h-3 rounded-sm ${color}`} />
+      <span>{cacheTokens}c</span>
+      {totalTokens != null && totalTokens > 0 && (
+        <span className="text-muted-foreground/40">{pct}%</span>
+      )}
+    </span>
+  );
+}