Initial release of game-compare-backends skill.

- Enables narrow comparison between two or three backend profiles when architectural uncertainty exists.
- Warns users that compare mode may cost more tokens and time than selecting a single route.
- Designed for greenfield projects with genuine uncertainty (e.g., UI vs world focus, 2D vs 3D preview).
- Not intended for obvious choices or post-architecture tweaks.
- Outputs a decision summary to docs/game-studio/backend-decision.md, detailing criteria, outcome, reasoning, and cost notes.