fix: duplicate messages in quiet mode (#680)

Addressing #600 and #664 (partially)

## Bug
Codex was staging duplicate items in output running when the same
response item appeared in both the streaming events. Specifically:

1. Items would be staged once when received as a
`response.output_item.done` event
2. The same items would be staged again when included in the final
`response.completed` payload

This duplication would result in each message being sent several times
in the quiet mode output.

## Changes
- Added a Set (`alreadyStagedItemIds`) to track items that have already
been staged
- Modified the `stageItem` function to check if an item's ID is already
in this set before staging it
- Added a regression test (`agent-dedupe-items.test.ts`) that verifies
items with the same ID are only staged once

## Testing
Like other tests, the included test creates a mock OpenAI stream that
emits the same message twice (once as an incremental event and once in
the final response) and verifies the item is only passed to `onItem`
once.

codex-cli/src/utils/agent/agent-loop.ts

@@ -46,6 +46,7 @@ export type CommandConfirmation = {
 };
 
 const alreadyProcessedResponses = new Set();
+const alreadyStagedItemIds = new Set<string>();
 
 type AgentLoopParams = {
   model: string;
@@ -562,6 +563,12 @@ export class AgentLoop {
           return;
         }
 
+        // Skip items we've already processed to avoid staging duplicates
+        if (item.id && alreadyStagedItemIds.has(item.id)) {
+          return;
+        }
+        alreadyStagedItemIds.add(item.id);
+
         // Store the item so the final flush can still operate on a complete list.
         // We'll nil out entries once they're delivered.
         const idx = staged.push(item) - 1;