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 + %)

.omo/drafts/workflow-engine-design.md

@@ -0,0 +1,55 @@
+# Dynamic Workflow Engine — Design
+
+## Architecture
+
+```
+User writes workflow JS file:
+.boocode/workflows/my-flow.js
+
+Workflow Runtime (apps/server)
+  ├── isolated-vm sandbox (or node:vm)
+  ├── API surface: agent(), parallel(), pipeline(), phase(), budget()
+  ├── Tool bridge → BooCode's existing tool set
+  ├── Workflow manager (concurrency, lifecycle)
+  ├── Resumability cache (SHA-256 of agent spec)
+  └── Catalog (built-in workflows: deep-research, review-code)
+
+Workflow execution:
+  1. User triggers workflow (slash command or Orchestrator panel)
+  2. File discovery finds .boocode/workflows/<name>.js
+  3. Sandbox compiles and executes the script
+  4. agent() calls go through tool bridge → existing inference pipeline
+  5. parallel() spawns concurrent agent calls (max 3 default)
+  6. Results stream via existing WS frames
+  7. Completed agents cached by hash for resume
+
+API Surface (Claude Code compatible):
+  agent(prompt, { label?, schema?, model?, capabilities?, max_tool_calls? })
+  parallel([() => agent(...), () => agent(...)])
+  pipeline(items, ...stages)
+  phase(title)
+  log(message)
+  budget.total / budget.spent() / budget.remaining()
+  args
+  workflow(name, args?)  — one level of nesting
+```
+
+## Implementation Plan
+
+### Phase 1: Core Runtime (this session)
+- Sandbox using Node's `vm` module (no extra deps)
+- `agent()` function that creates a task and waits for completion
+- Workflow file discovery
+- Basic workflow manager
+
+### Phase 2: Advanced Primitives
+- `parallel()` with concurrency limits
+- `pipeline()` streaming
+- `budget()` token tracking
+- Workflow resumability cache
+
+### Phase 3: UI + Polish
+- Integration with Orchestrator panel
+- Built-in workflow catalog
+- Workflow editor
+- Error recovery