Add managed worktrees to codex exec (#42652)

## What changed

- Add the experimental `worktrees` feature and a shared `--worktree` flag for new and forked `codex exec` sessions.
- Create each enabled session in a managed Git worktree, use that checkout as the session working directory, and bind the checkout to the new thread.
- Share the configured worktree pool with Desktop while leaving automatic cleanup disabled for CLI allocations.
- Reject unsupported commands, remote execution, ignored user configuration, ephemeral sessions, and use without the feature enabled before allocating a worktree.

## Testing

- Cover flag placement and inheritance, supported and rejected command combinations, worktree allocation and thread ownership, configuration gating, and compatibility with existing worktree-backed sessions.

GitOrigin-RevId: 011ff4639b09e8992c50d7b823df23e71798670e

codex-rs/core/config.schema.json

@@ -1056,6 +1056,9 @@
             "workspace_owner_usage_nudge": {
               "type": "boolean"
             },
+            "worktrees": {
+              "type": "boolean"
+            },
             "write_stdin_approval": {
               "type": "boolean"
             }
@@ -6407,6 +6410,9 @@
         "workspace_owner_usage_nudge": {
           "type": "boolean"
         },
+        "worktrees": {
+          "type": "boolean"
+        },
         "write_stdin_approval": {
           "type": "boolean"
         }