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/arc_monitor.rs

@@ -383,7 +383,6 @@ fn build_arc_monitor_message_item(
         | ResponseItem::CustomToolCallOutput { .. }
         | ResponseItem::ToolSearchOutput { .. }
         | ResponseItem::ImageGenerationCall { .. }
-        | ResponseItem::GhostSnapshot { .. }
         | ResponseItem::Compaction { .. }
         | ResponseItem::Other => None,
     }