dsh-gamer: Enabling DSH Agents to Play Games on dsh-gaming-platform

The DSH `dsh-gamer` plugin, via the "Gamer" preset, enables agents to play games automatically. It employs a ticket mechanism to execute matches precisely, supports spectator mode, and strictly limits network commands to maintain focused gameplay. The plugin should be installed with the preset, and local checkout is recommended. Agents can register, find games, enter rooms, and operate matches through commands, making it well suited for using inexpensive models to control costs.