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_remote.rs

@@ -145,14 +145,6 @@ async fn run_remote_compact_task_inner_impl(
     // compact endpoint. The checkpoint below records it separately from the next sampling request,
     // whose prompt will repeat current developer/context prefix items.
     let trace_input_history = history.raw_items().to_vec();
-    // Required to keep `/undo` available after compaction
-    let ghost_snapshots: Vec<ResponseItem> = history
-        .raw_items()
-        .iter()
-        .filter(|item| matches!(item, ResponseItem::GhostSnapshot { .. }))
-        .cloned()
-        .collect();
-
     let prompt_input = history.for_prompt(&turn_context.model_info.input_modalities);
     let tool_router = built_tools(
         sess.as_ref(),
@@ -204,9 +196,6 @@ async fn run_remote_compact_task_inner_impl(
     )
     .await;
 
-    if !ghost_snapshots.is_empty() {
-        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()),
@@ -290,7 +279,6 @@ fn should_keep_compacted_history_item(item: &ResponseItem) -> bool {
         | ResponseItem::CustomToolCallOutput { .. }
         | ResponseItem::WebSearchCall { .. }
         | ResponseItem::ImageGenerationCall { .. }
-        | ResponseItem::GhostSnapshot { .. }
         | ResponseItem::Other => false,
     }
 }