fix(tui): allow Enter when @ autocomplete has no matches

When @ autocomplete shows "No matching items", pressing Enter
should submit the prompt instead of doing nothing.

**Problem:**
- User types `@nonexistentfile` (text that doesn't match any files)
- Autocomplete displays "No matching items"
- Pressing Enter does nothing

**Fix:**
- Remove redundant e.preventDefault() when no matching items exist
- Autocomplete now closes with single Enter press
- Prompt submits immediately

Author: wanghanzhen

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

@@ -628,9 +628,15 @@ export function Autocomplete(props: {
             return
           }
           if (name === "return") {
-            select()
-            e.preventDefault()
-            return
+            const selected = options()[store.selected]
+            if (selected) {
+              select()
+              e.preventDefault()
+              return
+            }
+            if (options().length === 0) {
+              hide()
+            }
           }
           if (name === "tab") {
             const selected = options()[store.selected]