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/context_manager/history.rs

@@ -103,8 +103,7 @@ impl ContextManager {
     {
         for item in items {
             let item_ref = item.deref();
-            let is_ghost_snapshot = matches!(item_ref, ResponseItem::GhostSnapshot { .. });
-            if !is_api_message(item_ref) && !is_ghost_snapshot {
+            if !is_api_message(item_ref) {
                 continue;
             }
 
@@ -120,8 +119,6 @@ impl ContextManager {
     pub(crate) fn for_prompt(mut self, input_modalities: &[InputModality]) -> Vec<ResponseItem> {
         self.normalize_history(input_modalities);
         self.items
-            .retain(|item| !matches!(item, ResponseItem::GhostSnapshot { .. }));
-        self.items
     }
 
     /// Returns raw items in the history.
@@ -403,7 +400,6 @@ impl ContextManager {
             | ResponseItem::ImageGenerationCall { .. }
             | ResponseItem::CustomToolCall { .. }
             | ResponseItem::Compaction { .. }
-            | ResponseItem::GhostSnapshot { .. }
             | ResponseItem::Other => item.clone(),
         }
     }
@@ -492,7 +488,6 @@ fn is_api_message(message: &ResponseItem) -> bool {
         | ResponseItem::WebSearchCall { .. }
         | ResponseItem::ImageGenerationCall { .. }
         | ResponseItem::Compaction { .. } => true,
-        ResponseItem::GhostSnapshot { .. } => false,
         ResponseItem::Other => false,
     }
 }
@@ -534,7 +529,6 @@ static ORIGINAL_IMAGE_ESTIMATE_CACHE: LazyLock<BlockingLruCache<[u8; 20], Option
 
 pub(crate) fn estimate_response_item_model_visible_bytes(item: &ResponseItem) -> i64 {
     match item {
-        ResponseItem::GhostSnapshot { .. } => 0,
         ResponseItem::Reasoning {
             encrypted_content: Some(content),
             ..
@@ -691,7 +685,6 @@ fn is_model_generated_item(item: &ResponseItem) -> bool {
         ResponseItem::FunctionCallOutput { .. }
         | ResponseItem::ToolSearchOutput { .. }
         | ResponseItem::CustomToolCallOutput { .. }
-        | ResponseItem::GhostSnapshot { .. }
         | ResponseItem::Other => false,
     }
 }

codex-rs/core/src/context_manager/history_tests.rs

@@ -1,7 +1,6 @@
 use super::*;
 use base64::Engine;
 use base64::engine::general_purpose::STANDARD as BASE64_STANDARD;
-use codex_git_utils::GhostCommit;
 use codex_protocol::AgentPath;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::models::BaseInstructions;
@@ -594,22 +593,6 @@ fn for_prompt_clears_image_generation_result_when_images_are_unsupported() {
     );
 }
 
-#[test]
-fn get_history_for_prompt_drops_ghost_commits() {
-    let items = vec![ResponseItem::GhostSnapshot {
-        ghost_commit: GhostCommit::new(
-            "ghost-1".to_string(),
-            /*parent*/ None,
-            Vec::new(),
-            Vec::new(),
-        ),
-    }];
-    let history = create_history_with_items(items);
-    let modalities = default_input_modalities();
-    let filtered = history.for_prompt(&modalities);
-    assert_eq!(filtered, vec![]);
-}
-
 #[test]
 fn estimate_token_count_with_base_instructions_uses_provided_text() {
     let history = create_history_with_items(vec![assistant_msg("hello from history")]);