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/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;