[FEATURE]: allow specification of additional configuration directories via project-local configuration

Feature hasn't been suggested before.

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

Describe the enhancement you want to request

I would like to create a new project which is already set up to use a specific set of configuration, but I don't want to copy that configuration boilerplate into the project each time. The most straightforward way to do this would be to add an option to opencode.json so That I could create the new one with

{
  "$schema": "https://opencode.ai/config.json",
  "extraConfigDirs": [
    "/one/path",
    "/another/path",
    "/a/third/path"
  ]
}

And it would be as if I created a new project with a bunch of boilerplate skills and commands under .opencode, but without actual boilerplate.

This is most similar to #26051 (closed for inactivity), except that it asks that we support a config file setting rather than an environment variable. Both are good, but for different use-cases.