refactor: migrate test runner from vitest to bun:test

- Replace vitest with bun:test across all 8 packages (47 test files)
- vi.spyOn → spyOn, vi.restoreAllMocks() → mock.restore() (3 files)
- toHaveBeenCalledOnce → toHaveBeenCalledTimes(1) (bun:test compat)
- Delete all vitest.config.ts files
- Remove vitest from devDependencies
- Add preload.ts for process.exit mock (cli-workflow)
- Fix import ordering (biome check --write)

All tests pass. Closes #601

packages/workflow-util-agent/__tests__/session-cache.test.ts

@@ -1,7 +1,7 @@
+import { afterEach, beforeEach, describe, expect, test } from "bun:test";
 import { mkdir, readdir, readFile, rm, stat, writeFile } from "node:fs/promises";
 import { dirname, join } from "node:path";
 import type { ThreadId } from "@uncaged/workflow-protocol";
-import { afterEach, beforeEach, describe, expect, test } from "vitest";
 
 import { getCachedSessionId, getCachePath, setCachedSessionId } from "../src/session-cache.js";
 import { resolveStorageRoot } from "../src/storage.js";