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.
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