fix(opencode): export default plugin definition for opencode2 compatibility

[mengbo]

Fix OpenCode2 plugin contract mismatch by adding a default export with id + setup wrapper while preserving existing plugin logic.

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[kangal-bot]

Hi @mengbo, thanks for your interest in contributing.

Herdr does not accept unsolicited implementation pull requests from contributors who are not listed in .github/APPROVED_CONTRIBUTORS.

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