fix: deduplicate prompt history entries

Avoid adding duplicate entries to prompt history when the same input
is appended multiple times (e.g., clearing with ctrl+c then restoring
via history navigation and clearing again).

Author: ariane-emory

packages/opencode/src/cli/cmd/tui/component/prompt/history.tsx

@@ -82,6 +82,7 @@ export const { use: usePromptHistory, provider: PromptHistoryProvider } = create
         return store.history.at(store.index)
       },
       append(item: PromptInfo) {
+        if (store.history.at(-1)?.input === item.input) return
         const entry = clone(item)
         let trimmed = false
         setStore(