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/mod.rs

@@ -168,7 +168,6 @@ use crate::compact::collect_user_messages;
 use crate::config::Config;
 use crate::config::Constrained;
 use crate::config::ConstraintResult;
-use crate::config::GhostSnapshotConfig;
 use crate::config::StartedNetworkProxy;
 use crate::config::resolve_web_search_mode_for_turn;
 use crate::context_manager::ContextManager;
@@ -289,10 +288,7 @@ use crate::state::SessionState;
 use crate::stream_events_utils::HandleOutputCtx;
 #[cfg(test)]
 use crate::stream_events_utils::handle_output_item_done;
-use crate::tasks::GhostSnapshotTask;
 use crate::tasks::ReviewTask;
-use crate::tasks::SessionTask;
-use crate::tasks::SessionTaskContext;
 use crate::tools::network_approval::NetworkApprovalService;
 use crate::tools::network_approval::build_blocked_request_observer;
 use crate::tools::network_approval::build_network_policy_decider;
@@ -358,7 +354,6 @@ use codex_protocol::user_input::UserInput;
 use codex_tools::ToolsConfig;
 use codex_tools::ToolsConfigParams;
 use codex_utils_absolute_path::AbsolutePathBuf;
-use codex_utils_readiness::Readiness;
 use codex_utils_readiness::ReadinessFlag;
 #[cfg(test)]
 use codex_utils_stream_parser::ProposedPlanSegment;
@@ -2912,34 +2907,6 @@ impl Session {
         self.send_event(turn_context, event).await;
     }
 
-    async fn maybe_start_ghost_snapshot(
-        self: &Arc<Self>,
-        turn_context: Arc<TurnContext>,
-        cancellation_token: CancellationToken,
-    ) {
-        if !self.enabled(Feature::GhostCommit) {
-            return;
-        }
-        let token = match turn_context.tool_call_gate.subscribe().await {
-            Ok(token) => token,
-            Err(err) => {
-                warn!("failed to subscribe to ghost snapshot readiness: {err}");
-                return;
-            }
-        };
-
-        info!("spawning ghost snapshot task");
-        let task = GhostSnapshotTask::new(token);
-        Arc::new(task)
-            .run(
-                Arc::new(SessionTaskContext::new(self.clone())),
-                turn_context.clone(),
-                Vec::new(),
-                cancellation_token,
-            )
-            .await;
-    }
-
     /// Inject additional user input into the currently active turn.
     ///
     /// Returns the active turn id when accepted.

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,
                 };

codex-rs/core/src/session/turn_context.rs

@@ -1,4 +1,5 @@
 use super::*;
+use crate::config::GhostSnapshotConfig;
 use codex_model_provider::SharedModelProvider;
 use codex_model_provider::create_model_provider;
 use codex_protocol::models::AdditionalPermissionProfile;