update three.arcprize.org to arcprize.org

[solidmecha]

fixed links in env.example, readme, and base_url

[ianozsvald]

I've hit the issue where at the end of a run, the scorecard URL points at three.arcprize.org/scorecards which gives a 404. I had to fix it by changing HOST in .env to arcprize.org (removing three.). I did try the suggestion in this PR of changing ARC_BASE_URL but that didn't fix the issue. After changing HOST at the end of a run I get:

$ uv run main.py --agent=fastllm --game=ls20
...
2026-04-22 11:57:12,941 | INFO | View your scorecard online: https://arcprize.org/scorecards/cfef929e-45ff-4750-b727-d43af6e29240

and it all looks like it ran correctly.