fix: resolve test failures after shared packages migration

Changes:
- Move subprocess-manager tests to @automaker/platform package
  - Tests need to be co-located with source for proper mocking
  - Add vitest configuration to platform package
  - 17/17 platform tests pass

- Update server vitest.config.ts to alias @automaker/* packages
  - Resolve to source files for proper mocking in tests
  - Enables vi.mock() and vi.spyOn() to work correctly

- Fix security.test.ts imports
  - Update dynamic imports from @/lib/security.js to @automaker/platform
  - Module was moved to shared package

- Rewrite prompt-builder.test.ts
  - Use fs/promises mock instead of trying to spy on internal calls
  - 10/10 tests pass

Test Results:
✅ Server: 536/536 tests pass
✅ Platform: 17/17 tests pass

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

libs/platform/package.json

@@ -6,7 +6,9 @@
   "types": "dist/index.d.ts",
   "scripts": {
     "build": "tsc",
-    "watch": "tsc --watch"
+    "watch": "tsc --watch",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "keywords": ["automaker", "platform"],
   "author": "",
@@ -15,6 +17,7 @@
   },
   "devDependencies": {
     "@types/node": "^22.10.5",
-    "typescript": "^5.7.3"
+    "typescript": "^5.7.3",
+    "vitest": "^3.1.4"
   }
 }