feat: Add agent color config

[0xrin1]

This PR adds the ability to customize agent colors in the opencode configuration.

You can specify custom colors for agents in your opencode.json:

{
  $schema: https://opencode.ai/config.json,
  agent: {
    glm-4.6: {
      model: openrouter/zhipuai/glm-4-plus,
      color: #3B82F6,
      ...
    },
    chkn: { 
      model: openrouter/gpt-5-high,
      color: #DC2626,
      ...
    },
    plan: { 
      model: openrouter/gpt-5-high,
      mode: primary,
      color: #A855F7,
      ...
    },
    claude: { 
      model: anthropic/claude-sonnet-4-5-20250929,
      color: #D97706,
      ...
    }
  }
}

The colors are specified as hex color codes and will be applied to agent indicators in the CLI/TUI output, making it easier to distinguish between different agent types at a glance.

[0xrin1]

Relates to issue #1973