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/config.schema.json

@@ -766,16 +766,16 @@
       "additionalProperties": false,
       "properties": {
         "disable_warnings": {
-          "description": "Disable all ghost snapshot warning events.",
+          "description": "Legacy no-op setting retained for compatibility.",
           "type": "boolean"
         },
         "ignore_large_untracked_dirs": {
-          "description": "Ignore untracked directories that contain this many files or more. (Still emits a warning unless warnings are disabled.)",
+          "description": "Legacy no-op setting retained for compatibility.",
           "format": "int64",
           "type": "integer"
         },
         "ignore_large_untracked_files": {
-          "description": "Exclude untracked files larger than this many bytes from ghost snapshots.",
+          "description": "Legacy no-op setting retained for compatibility.",
           "format": "int64",
           "type": "integer"
         }
@@ -2824,7 +2824,7 @@
         }
       ],
       "default": null,
-      "description": "Settings for ghost snapshots (used for undo)."
+      "description": "Compatibility-only settings retained so legacy `ghost_snapshot` config still loads."
     },
     "hide_agent_reasoning": {
       "description": "When set to `true`, `AgentReasoning` events will be hidden from the UI/output. Defaults to `false`.",