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/otel/src/events/session_telemetry.rs

@@ -1078,7 +1078,6 @@ impl SessionTelemetry {
             ResponseItem::CustomToolCallOutput { .. } => "custom_tool_call_output".into(),
             ResponseItem::WebSearchCall { .. } => "web_search_call".into(),
             ResponseItem::ImageGenerationCall { .. } => "image_generation_call".into(),
-            ResponseItem::GhostSnapshot { .. } => "ghost_snapshot".into(),
             ResponseItem::Compaction { .. } => "compaction".into(),
             ResponseItem::Other => "other".into(),
         }