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/tasks/mod.rs

@@ -1,5 +1,4 @@
 mod compact;
-mod ghost_snapshot;
 mod regular;
 mod review;
 mod undo;
@@ -54,7 +53,6 @@ use codex_protocol::user_input::UserInput;
 use codex_features::Feature;
 use codex_protocol::models::ContentItem;
 pub(crate) use compact::CompactTask;
-pub(crate) use ghost_snapshot::GhostSnapshotTask;
 pub(crate) use regular::RegularTask;
 pub(crate) use review::ReviewTask;
 pub(crate) use undo::UndoTask;