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/config/mod.rs

@@ -124,7 +124,27 @@ pub use network_proxy_spec::NetworkProxySpec;
 pub use network_proxy_spec::StartedNetworkProxy;
 pub(crate) use permissions::resolve_permission_profile;
 
-pub use codex_git_utils::GhostSnapshotConfig;
+const DEFAULT_IGNORE_LARGE_UNTRACKED_DIRS: i64 = 200;
+const DEFAULT_IGNORE_LARGE_UNTRACKED_FILES: i64 = 10 * 1024 * 1024;
+
+/// Compatibility-only config retained so legacy `ghost_snapshot` settings
+/// continue to load even though snapshots are no longer produced.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct GhostSnapshotConfig {
+    pub ignore_large_untracked_files: Option<i64>,
+    pub ignore_large_untracked_dirs: Option<i64>,
+    pub disable_warnings: bool,
+}
+
+impl Default for GhostSnapshotConfig {
+    fn default() -> Self {
+        Self {
+            ignore_large_untracked_files: Some(DEFAULT_IGNORE_LARGE_UNTRACKED_FILES),
+            ignore_large_untracked_dirs: Some(DEFAULT_IGNORE_LARGE_UNTRACKED_DIRS),
+            disable_warnings: false,
+        }
+    }
+}
 
 /// Maximum number of bytes of the documentation that will be embedded. Larger
 /// files are *silently truncated* to this size so we do not take up too much of
@@ -655,7 +675,8 @@ pub struct Config {
     /// Default: `300000` (5 minutes).
     pub background_terminal_max_timeout: u64,
 
-    /// Settings for ghost snapshots (used for undo).
+    /// Compatibility-only settings retained for legacy `ghost_snapshot`
+    /// config loading.
     pub ghost_snapshot: GhostSnapshotConfig,
 
     /// Settings specific to the task-path-based multi-agent tool surface.