feat(KB-044): add test coverage reporting with vitest coverage-v8

- Add @vitest/coverage-v8 to all workspace packages
- Configure vitest coverage settings in all vitest.config.ts files
- Align vitest versions across packages for consistency
- Add 'test:coverage' script to root package.json
- Verify all tests pass with coverage reporting enabled

packages/core/package.json

@@ -22,6 +22,7 @@
   },
   "devDependencies": {
     "@types/node": "^25.5.0",
+    "@vitest/coverage-v8": "^3.1.0",
     "typescript": "^5.7.0",
     "vitest": "^3.1.0"
   },

packages/core/vitest.config.ts

@@ -3,5 +3,12 @@ import { defineConfig } from "vitest/config";
 export default defineConfig({
   test: {
     include: ["src/**/*.test.ts"],
+    coverage: {
+      enabled: false,
+      reporter: ["text", "html", "json"],
+      reportsDirectory: "./coverage",
+      include: ["src/**/*.ts"],
+      exclude: ["**/*.test.ts", "**/*.d.ts", "dist/**"],
+    },
   },
 });