feat: add model aliases

[pakerfeldt]

Closes #3439

This is my first PR to OpenCode. I've done my best to follow the contributing guidelines and code style, but I fully expect maintainers may want adjustments to the code, documentation, or both. Happy to iterate based on feedback.

Summary

Adds model_aliases config option to map short names to provider/model strings. Aliases work anywhere a model ID is accepted.
Example

{
  model_aliases: {
    cheap: anthropic/claude-3-haiku,
    acme/reasoning: openai/gpt-5
  },
  model: cheap
}

One concern: I introduced an async resolveModel() function (since it needs config access). Most call sites were already async, but I'd appreciate a sanity check that this doesn't cause issues I'm not aware of.

Verification

• Added 12 unit tests for resolveAlias() and resolveModel()
• Tested manually

[github-actions]

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No duplicate PRs found

[github-actions]

Closing this pull request because it has had no updates for more than 60 days. If you plan to continue working on it, feel free to reopen or open a new PR.