Make action a thin wrapper around game-ci/unity-engine-core

Delegates build logic to the extracted implementation in
game-ci/unity-engine-core instead of maintaining a local copy, per
game-ci/roadmap#11 workstream 2 (Option A) — third and final engine
repo to make this move, after unity-activate and unity-test-runner.
src/model/* removed; build/test coverage (including the index.ts
orchestration integration tests) now lives in the destination repo.

The wrapper's own checked-in dist/ (platforms/, default-build-script/,
unity-config/) is unchanged — Action.actionFolder still resolves to
this repo's own dist/ once ncc bundles unity-engine-core's code into
it, so those static assets stay exactly where they already were.

vitest.config.mts

@@ -6,6 +6,7 @@ export default defineConfig({
     globals: true,
     setupFiles: ['./src/test/setup.ts'],
     include: ['src/**/*.test.ts'],
+    passWithNoTests: true,
     coverage: {
       provider: 'istanbul',
       reporter: ['text', 'html', 'lcov'],