Remove ghost snapshots (#19481)

## Summary
- Remove `ghost_snapshot` / `GhostCommit` from the Responses API surface
and generated SDK/schema artifacts.
- Keep legacy config loading compatible, but make undo a no-op that
reports the feature is unavailable.
- Clean up core history, compaction, telemetry, rollout, and tests to
stop carrying ghost snapshot items.

## Testing
- Unit tests passed for `codex-protocol`, `codex-core` targeted undo and
compaction flows, `codex-rollout`, and `codex-app-server-protocol`.
- Regenerated config and app-server schemas plus Python SDK artifacts
and verified they match the checked-in outputs.

codex-rs/core/src/compact.rs

@@ -265,12 +265,6 @@ async fn run_compact_task_inner_impl(
         new_history =
             insert_initial_context_before_last_real_user_or_summary(new_history, initial_context);
     }
-    let ghost_snapshots: Vec<ResponseItem> = history_items
-        .iter()
-        .filter(|item| matches!(item, ResponseItem::GhostSnapshot { .. }))
-        .cloned()
-        .collect();
-    new_history.extend(ghost_snapshots);
     let reference_context_item = match initial_context_injection {
         InitialContextInjection::DoNotInject => None,
         InitialContextInjection::BeforeLastUserMessage => Some(turn_context.to_turn_context_item()),