Add opencode-provider-alias to ecosystem docs

[baranwang]

Add opencode-provider-alias to the ecosystem Plugins table so users can discover the OpenCode provider/model alias plugin.

[github-actions]

This issue doesn't fully meet our contributing guidelines.

What needs to be fixed:

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[github-actions]

This issue has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

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