Vouch request: OpenCode runtime implementation (#1260)

[sonupreetam]

I'd like to contribute to the OpenCode runtime integration tracked in #1260.

Planned first PR: Register "opencode" in ValidRuntimes() and Resolve(), create a stub OpenCodeRuntime in opencode_runtime.go with no-op methods (same pattern as DummyRuntime). This validates the config-to-registry path end-to-end.

Context: I posted a codebase readiness assessment on #1260 and #1935 covering the implementation sequence, existing infrastructure, and known friction points.

Background: I work on the council-review-action in unbound-force, which already runs OpenCode in CI production for AI code review.

[waynesun09]

/vouch

[github-actions[bot]]

@sonupreetam has been vouched by @waynesun09.

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