refactor(workflow): migrate downstream packages to workflow-runtime (Phase 2+3)

- Verify createWorkflow in runtime has zero I/O imports
- Migrate agent-cursor, agent-hermes to pure workflow-runtime dependency
- Migrate agent-llm, util-agent, templates to dual dependency
  (runtime for types, engine for CAS/merkle/buildDescriptor)
- All 377 tests passing

Refs #121, relates #123 #124

packages/workflow-util-agent/__tests__/build-agent-prompt.test.ts

@@ -2,7 +2,8 @@ import { afterEach, beforeEach, describe, expect, test } from "bun:test";
 import { mkdtemp, rm } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { createCasStore, putContentMerkleNode, START, type ThreadContext } from "@uncaged/workflow";
+import { createCasStore, putContentMerkleNode } from "@uncaged/workflow";
+import { START, type ThreadContext } from "@uncaged/workflow-runtime";
 
 import { buildAgentPrompt } from "../src/index.js";