v1.21.0

• Use icy_sixel for better terminal rendering.
• Support gemini-3-pro.
• Support gpt-5.1 / gpt-5.1-chat / gpt-5-chat.
• Set default model to gpt-5.1-chat.
• Mask strings when printing steps.
• Add HAI_IMAGE_HEIGHT env var.
• Update cost for gemini-2.5-{flash,pro}.
• Allow underscores in named functions !fn.
• Fix evaluation of truecolor terminal support.