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- Mark workflow-as-agent plan as outdated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-register/README.md

@@ -0,0 +1,38 @@
+# @uncaged/workflow-register
+
+Bundle validation, dynamic export extraction, registry YAML, and model/provider resolution.
+
+## What This Package Does
+
+It validates workflow `.esm.js` bundles, extracts `descriptor` / `run` exports at runtime, reads and writes `workflow.yaml`, and resolves which LLM endpoint/model to use from registry config (`resolveModel`, `splitProviderModelRef`).
+
+## Key Exports
+
+From `src/index.ts`:
+
+- **Bundle:** `buildDescriptor`, `importWorkflowBundleModule`, `validateWorkflowBundle`, `ensureUncagedWorkflowSymlink`, `extractBundleExports`, `stringifyWorkflowDescriptor`, `validateWorkflowDescriptor`
+- **Bundle types:** `ExtractBundleExportsOptions`, `ExtractedBundleExports`, `WorkflowBundleValidationInput`, `WorkflowDescriptor`, `WorkflowRoleDescriptor`, `WorkflowRoleSchema`
+- **Registry:** `getRegisteredWorkflow`, `listRegisteredWorkflowNames`, `parseWorkflowRegistryYaml`, `readWorkflowRegistry`, `registerWorkflowVersion`, `rollbackWorkflowToHistoryHash`, `stringifyWorkflowRegistryYaml`, `unregisterWorkflow`, `workflowRegistryPath`, `writeWorkflowRegistry`
+- **Registry types:** `WorkflowConfig`, `WorkflowHistoryEntry`, `WorkflowRegistryEntry`, `WorkflowRegistryFile`
+- **Config:** `resolveModel`, `splitProviderModelRef`, types `ProviderConfig`, `ResolvedModel`
+
+## Dependencies
+
+- **Workspace:** `@uncaged/workflow-protocol`, `@uncaged/workflow-util`
+- **Peer:** `acorn`, `yaml`, `zod` ^4 — parsing/validation at runtime for consumers
+
+## Usage
+
+```typescript
+import { readFile } from "node:fs/promises";
+import { readWorkflowRegistry, validateWorkflowBundle } from "@uncaged/workflow-register";
+import { getDefaultWorkflowStorageRoot } from "@uncaged/workflow-util";
+
+const reg = await readWorkflowRegistry(getDefaultWorkflowStorageRoot());
+if (!reg.ok) throw new Error(reg.error.message);
+
+const path = "./my.esm.js";
+const source = await readFile(path, "utf8");
+const v = validateWorkflowBundle({ filePath: path, source });
+if (!v.ok) throw new Error(v.error);
+```