feat: add toggle to control whether cleared prompts are saved to history

Adds a toggle command in the System category that allows users to enable
or disable saving cleared prompts to history. The feature is disabled by
default to preserve existing behavior.

When enabled via the command palette ("Include cleared prompts in history"),
pressing Ctrl+C will save the current prompt to history before clearing it,
allowing users to navigate back with arrow keys.

The setting persists in kv.json.

packages/opencode/src/cli/cmd/tui/app.tsx

@@ -582,6 +582,15 @@ function App() {
         dialog.clear()
       },
     },
+    {
+      title: kv.get("clear_prompt_save_history", false) ? "Don't include cleared prompts in history" : "Include cleared prompts in history",
+      value: "app.toggle.clear_prompt_history",
+      category: "System",
+      onSelect: (dialog) => {
+        kv.set("clear_prompt_save_history", !kv.get("clear_prompt_save_history", false))
+        dialog.clear()
+      },
+    },
   ])
 
   createEffect(() => {

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

@@ -832,10 +832,12 @@ export function Prompt(props: PromptProps) {
                   // If no image, let the default paste behavior continue
                 }
                 if (keybind.match("input_clear", e) && store.prompt.input !== "") {
-                  history.append({
-                    ...store.prompt,
-                    mode: store.mode,
-                  })
+                  if (kv.get("clear_prompt_save_history", false)) {
+                    history.append({
+                      ...store.prompt,
+                      mode: store.mode,
+                    })
+                  }
                   input.clear()
                   input.extmarks.clear()
                   setStore("prompt", {