Migrate sequentialthinking and filesystem servers from Jest to Vitest

- Replace Jest dependencies with Vitest and @vitest/coverage-v8
- Update test scripts to use 'vitest run --coverage'
- Create vitest.config.ts for both servers with node environment and coverage settings
- Update all test files:
  - Change imports from '@jest/globals' to 'vitest'
  - Replace jest.mock() with vi.mock()
  - Replace jest.fn() with vi.fn()
  - Update mock clearing/restoring to use vi methods
- Remove jest.config.cjs files
- All 151 tests passing (24 in sequentialthinking, 127 in filesystem)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/sequentialthinking/__tests__/lib.test.ts

@@ -1,8 +1,8 @@
-import { describe, it, expect, beforeEach, jest } from '@jest/globals';
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
 import { SequentialThinkingServer, ThoughtData } from '../lib.js';
 
-// Mock chalk to avoid ESM issues in Jest
-jest.mock('chalk', () => {
+// Mock chalk to avoid ESM issues
+vi.mock('chalk', () => {
   const chalkMock = {
     yellow: (str: string) => str,
     green: (str: string) => str,
@@ -10,7 +10,6 @@ jest.mock('chalk', () => {
   };
   return {
     default: chalkMock,
-    __esModule: true,
   };
 });