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/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -5813,38 +5813,6 @@
       "title": "GetAccountResponse",
       "type": "object"
     },
-    "GhostCommit": {
-      "description": "Details of a ghost commit created from a repository state.",
-      "properties": {
-        "id": {
-          "type": "string"
-        },
-        "parent": {
-          "type": [
-            "string",
-            "null"
-          ]
-        },
-        "preexisting_untracked_dirs": {
-          "items": {
-            "type": "string"
-          },
-          "type": "array"
-        },
-        "preexisting_untracked_files": {
-          "items": {
-            "type": "string"
-          },
-          "type": "array"
-        }
-      },
-      "required": [
-        "id",
-        "preexisting_untracked_dirs",
-        "preexisting_untracked_files"
-      ],
-      "type": "object"
-    },
     "GitInfo": {
       "properties": {
         "branch": {
@@ -9797,26 +9765,6 @@
           "title": "ImageGenerationCallResponseItem",
           "type": "object"
         },
-        {
-          "properties": {
-            "ghost_commit": {
-              "$ref": "#/definitions/GhostCommit"
-            },
-            "type": {
-              "enum": [
-                "ghost_snapshot"
-              ],
-              "title": "GhostSnapshotResponseItemType",
-              "type": "string"
-            }
-          },
-          "required": [
-            "ghost_commit",
-            "type"
-          ],
-          "title": "GhostSnapshotResponseItem",
-          "type": "object"
-        },
         {
           "properties": {
             "encrypted_content": {