fix: make plugin entry-point async to permit await import()

[Copilot]

await import("./grok_agent_plugin") was called inside a non-async exported function, which is a syntax error in JS/TS.

Change

// before
export default function (api: any) {

// after
export default async function (api: any) {

The dynamic import on line 219 is now syntactically valid within the async context.

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