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/session/turn.rs

@@ -356,8 +356,6 @@ pub(crate) async fn run_turn(
     track_turn_resolved_config_analytics(&sess, &turn_context, &input).await;
 
     let skills_outcome = Some(turn_context.turn_skills.outcome.as_ref());
-    sess.maybe_start_ghost_snapshot(Arc::clone(&turn_context), cancellation_token.child_token())
-        .await;
     let mut last_agent_message: Option<String> = None;
     let mut stop_hook_active = false;
     // Although from the perspective of codex.rs, TurnDiffTracker has the lifecycle of a Task which contains
@@ -1951,7 +1949,6 @@ async fn try_run_sampling_request(
                     | ResponseItem::ToolSearchOutput { .. }
                     | ResponseItem::WebSearchCall { .. }
                     | ResponseItem::ImageGenerationCall { .. }
-                    | ResponseItem::GhostSnapshot { .. }
                     | ResponseItem::Compaction { .. }
                     | ResponseItem::Other => false,
                 };