docs: update architecture docs and package READMEs for post-split structure

- Rewrite docs/architecture.md with 15-package map, dependency graph, updated engine paths
- Update CLAUDE.md monorepo structure section
- Add READMEs for: workflow-protocol, workflow-runtime, workflow-util, workflow-cas, workflow-register, workflow-execute, workflow-reactor
- Fix agent READMEs: update deps from @uncaged/workflow to actual packages
- Mark workflow-as-agent plan as outdated

Fixes #153

小橘 <xiaoju@shazhou.work>

packages/workflow-runtime/README.md

@@ -0,0 +1,28 @@
+# @uncaged/workflow-runtime
+
+Workflow author API: `createWorkflow` plus re-exports of protocol workflow types.
+
+## What This Package Does
+
+Bundle code imports `createWorkflow` to turn a `WorkflowDefinition` plus `AgentBinding` into a `WorkflowFn` generator. It re-exports the protocol types and constants most authors need so workflows rarely import `@uncaged/workflow-protocol` directly.
+
+## Key Exports
+
+From `src/index.ts`:
+
+- **Functions:** `createWorkflow`, `ok`, `err`
+- **Types:** `AgentBinding`, `AgentContext`, `AgentFn`, `CasStore`, `ExtractContext`, `ExtractFn`, `LlmProvider`, `Moderator`, `ModeratorContext`, `Result`, `RoleDefinition`, `RoleMeta`, `RoleOutput`, `RoleStep`, `StartStep`, `ThreadContext`, `WorkflowCompletion`, `WorkflowDefinition`, `WorkflowDescriptor`, `WorkflowFn`, `WorkflowResult`, `WorkflowRoleDescriptor`, `WorkflowRoleSchema`, `WorkflowRuntime`
+- **Constants:** `END`, `START`
+
+## Dependencies
+
+- **Workspace:** `@uncaged/workflow-protocol` — contract types and helpers
+- **Peer:** `zod` ^4 — matches schema usage on role definitions
+
+## Usage
+
+```typescript
+import { createWorkflow, type WorkflowDefinition, type AgentBinding } from "@uncaged/workflow-runtime";
+
+export const run = createWorkflow(myDefinition, myBinding);
+```