test: migrate tests from centralized mjs to colocated TypeScript

Move test files from tests/ directory to colocate with source code,
convert from .mjs to .ts using tsx runner, add workspaces and npm
cache to CI workflow.

package.json

@@ -2,8 +2,14 @@
   "name": "baoyu-skills",
   "private": true,
   "type": "module",
+  "workspaces": [
+    "packages/*"
+  ],
   "scripts": {
-    "test": "node --test",
-    "test:coverage": "node --experimental-test-coverage --test"
+    "test": "node --import tsx --test",
+    "test:coverage": "node --import tsx --experimental-test-coverage --test"
+  },
+  "devDependencies": {
+    "tsx": "^4.20.5"
   }
 }