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/config/src/config_toml.rs

@@ -362,7 +362,8 @@ pub struct ConfigToml {
     /// Suppress warnings about unstable (under development) features.
     pub suppress_unstable_features_warning: Option<bool>,
 
-    /// Settings for ghost snapshots (used for undo).
+    /// Compatibility-only settings retained so legacy `ghost_snapshot`
+    /// config still loads.
     #[serde(default)]
     pub ghost_snapshot: Option<GhostSnapshotToml>,
 
@@ -629,14 +630,13 @@ impl From<ToolsToml> for Tools {
 #[derive(Serialize, Deserialize, Debug, Clone, Default, PartialEq, Eq, JsonSchema)]
 #[schemars(deny_unknown_fields)]
 pub struct GhostSnapshotToml {
-    /// Exclude untracked files larger than this many bytes from ghost snapshots.
+    /// Legacy no-op setting retained for compatibility.
     #[serde(alias = "ignore_untracked_files_over_bytes")]
     pub ignore_large_untracked_files: Option<i64>,
-    /// Ignore untracked directories that contain this many files or more.
-    /// (Still emits a warning unless warnings are disabled.)
+    /// Legacy no-op setting retained for compatibility.
     #[serde(alias = "large_untracked_dir_warning_threshold")]
     pub ignore_large_untracked_dirs: Option<i64>,
-    /// Disable all ghost snapshot warning events.
+    /// Legacy no-op setting retained for compatibility.
     pub disable_warnings: Option<bool>,
 }