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/__tests__/frontmatter-markdown.test.ts

@@ -1,4 +1,4 @@
-import { describe, expect, it } from "vitest";
+import { describe, expect, it } from "bun:test";
 import type { AgentFrontmatter } from "../src/index.js";
 import { parseFrontmatterMarkdown, validateFrontmatter } from "../src/index.js";