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.

package.json

@@ -32,16 +32,7 @@
     "node": ">=18.x"
   },
   "dependencies": {
-    "@actions/cache": "^4.1.0",
-    "@actions/core": "^1.11.1",
-    "@actions/exec": "^1.1.1",
-    "@actions/github": "^6.0.1",
-    "md5": "^2.3.0",
-    "nanoid": "^3.3.12",
-    "semver": "^7.7.4",
-    "ts-md5": "^1.3.1",
-    "unity-changeset": "^3.1.0",
-    "yaml": "^2.8.4"
+    "@game-ci/unity-engine-core": "game-ci/unity-engine-core#main"
   },
   "devDependencies": {
     "@types/node": "^17.0.23",