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

package.json

@@ -28,7 +28,6 @@
     "@uncaged/workflow-agent-hermes": "workspace:*",
     "bun-types": "^1.3.13",
     "typescript": "^5.8.3",
-    "vitest": "^4.1.7",
     "yaml": "^2.9.0"
   },
   "repository": {