feat(harness-opencode): support native OpenCode configuration (#19924)

Supersedes #19671, recreated at maintainer request with a clean,
GitHub-verified commit history.

## Background

OpenCode supports per-agent model selection and other native settings,
but `@ai-sdk/harness-opencode` only exposes adapter-level options. As a
result, callers cannot configure OpenCode subagents independently from
the parent agent.

## Summary

- add an optional `openCodeConfig` adapter setting
- pass it through prompt, resume, and compaction bridge starts
- merge it beneath adapter-managed safety and connectivity settings
- document per-agent model configuration

## End-to-End Verification

Verified in a harness-backed chat with an OpenAI parent model and an
OpenCode `general` subagent configured through Vercel AI Gateway to use
Anthropic Claude Haiku. The delegated task completed and its OpenCode
checkpoint recorded the configured child model independently from the
parent.

## Checklist

- [x] All commits are signed (PRs with unsigned commits cannot be
merged)
- [x] Tests have been added / updated (for bug fixes / features)
- [x] Documentation has been added / updated (for bug fixes / features)
- [x] A _patch_ changeset for relevant packages has been added (for bug
fixes / features - run `pnpm changeset` in the project root)
- [x] I have reviewed this pull request (self-review)

## Related Issues

Closes #19672