[FEATURE]: Composable configs

[eunjae-lee]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Hi team, thanks for the amazing product.

At the global level we have ~/.config/opencode/config.json. It'd be awesome if we can compose other configs into this. So the config.json can become like

{
  ...
  "extends": [
    // Extend from a local config
    "~/workspace/my-opencode-config",

    // Extend from a git repository
    'github:someone/awesome-opencode-config',

    // Extend from a git repository
    'github:someone/awesome-opencode-config#v1', // we can specify a tag via #v1
  ]
}

As the opencode community grows, this composable config will be really useful. Of course, there's a problem of "how to merge many configs if they have conflicting keys", but let me create an issue here to initiate the discussion.

I hope others find this request useful.