[FEATURE]: Expose @opentui/three to external TUI plugins

Feature hasn't been suggested before.

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Describe the enhancement you want to request

External TUI plugins can use host-provided OpenTUI modules, but OpenCode does not currently register @opentui/three in its runtime module map. A file plugin using ThreeRenderable therefore cannot use the host's Three/OpenTUI instances.

Would you accept adding the matching @opentui/three version to OpenCode and merging its runtimeModules into ensureRuntimePluginSupport({ additional })? This would expose the existing OpenTUI capability through the plugin system; it would not add a Three-specific sidebar API or any built-in scene.

I tested this locally with an external plugin rendering an animated, transparent eight-row scene in sidebar_content. A focused loader test verifies that the external plugin imports THREE and ThreeRenderable from the host runtime.

The install footprint includes @opentui/three and its three, jimp, and optional bun-webgpu dependencies. A companion OpenTUI fix can avoid evaluating Jimp for asset-free scenes under OpenCode's browser condition; the underlying Jimp browser-export problem is tracked in https://github.com/jimp-dev/jimp/issues/1402.