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/git-utils/README.md

@@ -1,21 +1,17 @@
 # codex-git-utils
 
-Helpers for interacting with git, including patch application and worktree
-snapshot utilities. The crate also exposes a lightweight baseline API for
-internal directories that use git only as a resettable diff mechanism:
-`ensure_git_baseline_repository` preserves a usable `root/.git` baseline or
-creates one when it is missing or unusable, `reset_git_repository` replaces
-`root/.git` with a fresh one-commit baseline, and `diff_since_latest_init`
-returns structured file changes plus a unified diff from that baseline to the
-current directory contents.
+Helpers for interacting with git, including patch application. The crate also
+exposes a lightweight baseline API for internal directories that use git only
+as a resettable diff mechanism: `ensure_git_baseline_repository` preserves a
+usable `root/.git` baseline or creates one when it is missing or unusable,
+`reset_git_repository` replaces `root/.git` with a fresh one-commit baseline,
+and `diff_since_latest_init` returns structured file changes plus a unified
+diff from that baseline to the current directory contents.
 
 ```rust,no_run
 use std::path::Path;
 
-use codex_git_utils::{
-    apply_git_patch, create_ghost_commit, restore_ghost_commit, ApplyGitRequest,
-    CreateGhostCommitOptions,
-};
+use codex_git_utils::{apply_git_patch, ApplyGitRequest};
 
 let repo = Path::new("/path/to/repo");
 
@@ -27,13 +23,4 @@ let request = ApplyGitRequest {
     preflight: false,
 };
 let result = apply_git_patch(&request)?;
-
-// Capture the current working tree as an unreferenced commit.
-let ghost = create_ghost_commit(&CreateGhostCommitOptions::new(repo))?;
-
-// Later, undo back to that state.
-restore_ghost_commit(repo, &ghost)?;
 ```
-
-Pass a custom message with `.message("…")` or force-include ignored files with
-`.force_include(["ignored.log".into()])`.