refactor: extract @uncaged/workflow-util-agent + smart prompt

- New package: spawn-cli + build-agent-prompt shared utils
- Smart prompt: start + meta summaries for middle steps + last step full
- Cursor/Hermes adapters now import from util-agent (no duplicate code)
- 109 tests pass, biome clean

Closes #14
小橘 <xiaoju@shazhou.work>

packages/workflow-agent-hermes/__tests__/hermes-agent.test.ts

@@ -1,19 +1,5 @@
 import { describe, expect, test } from "bun:test";
-import { START, type ThreadContext } from "@uncaged/workflow";
-
-import { buildAgentPrompt, createHermesAgent, validateHermesAgentConfig } from "../src/index.js";
-
-function makeCtx(): ThreadContext {
-  return {
-    start: {
-      role: START,
-      content: "plan the migration",
-      meta: { maxRounds: 8 },
-      timestamp: 1,
-    },
-    steps: [],
-  };
-}
+import { createHermesAgent, validateHermesAgentConfig } from "../src/index.js";
 
 describe("validateHermesAgentConfig", () => {
   test("accepts valid config", () => {
@@ -36,14 +22,6 @@ describe("validateHermesAgentConfig", () => {
   });
 });
 
-describe("buildAgentPrompt", () => {
-  test("includes system and thread start", () => {
-    const text = buildAgentPrompt(makeCtx(), "You are a planner.");
-    expect(text).toContain("You are a planner.");
-    expect(text).toContain("plan the migration");
-  });
-});
-
 describe("createHermesAgent", () => {
   test("returns an AgentFn", () => {
     const agent = createHermesAgent({