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- Mark workflow-as-agent plan as outdated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

@@ -0,0 +1,32 @@
+# @uncaged/workflow-util
+
+Shared utilities: encoding, IDs, logging, storage paths, and ref-field normalization.
+
+## What This Package Does
+
+It provides filesystem-safe Base32 and ULID generation, the structured logger used across packages, helpers for the default workflow data directory and global CAS path, and utilities to merge/normalize `refs` on steps. It re-exports `ok`/`err` from protocol for convenience.
+
+## Key Exports
+
+From `src/index.ts`:
+
+- **Base32:** `CROCKFORD_BASE32_ALPHABET`, `decodeCrockfordBase32Bits`, `decodeCrockfordToUint64`, `encodeCrockfordBase32Bits`, `encodeUint64AsCrockford`
+- **Logger:** `createLogger`
+- **Refs:** `mergeRefsWithContentHash`, `normalizeRefsField`
+- **Result:** `ok`, `err` (from `@uncaged/workflow-protocol`)
+- **Paths:** `getDefaultWorkflowStorageRoot`, `getGlobalCasDir`
+- **ULID:** `generateUlid`
+- **Types:** `CreateLoggerOptions`, `LogFn`, `LoggerSink`, `Result`
+
+## Dependencies
+
+- **Workspace:** `@uncaged/workflow-protocol` — `Result` and shared types used by helpers
+
+## Usage
+
+```typescript
+import { createLogger, getDefaultWorkflowStorageRoot, generateUlid } from "@uncaged/workflow-util";
+
+const log = createLogger();
+log("4KNMR2PX", "example");
+```