Add Unity AI Assistant package + editor settings migration

Adds com.unity.ai.assistant 2.12.0-pre.2 (+ transitive com.unity.ai.inference 2.6.1 and the com.unity.dt.app-ui EditorBuildSettings config) to the package manifest/lockfile, plus the package's ProjectSettings/Packages/com.unity.ai.assistant/Settings.json. ProjectSettings.asset migrated serializedVersion 28->29 (new iOS thermal-FPS / Android system-bar fields) by the editor. Editor/package tooling only; committed separately from the class-switch dev tool to keep concerns isolated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Packages/manifest.json

@@ -1,6 +1,8 @@
 {
   "dependencies": {
     "com.coplaydev.unity-mcp": "https://github.com/CoplayDev/unity-mcp.git?path=/MCPForUnity#main",
+    "com.unity.ai.assistant": "2.12.0-pre.2",
+    "com.unity.ai.inference": "2.6.1",
     "com.unity.ai.navigation": "2.0.13",
     "com.unity.charactercontroller": "1.4.2",
     "com.unity.cinemachine": "3.1.6",