Allow trusted symlinks beneath CODEX_HOME on macOS (#42716)

## What changed

- Add the macOS-only `allow_symlinked_codex_home` user setting. When enabled in the execution host's `$CODEX_HOME/config.toml`, writable roots at or beneath that home may traverse symlinks.
- Keep the exception disabled by default and prevent project config, command-line overrides, and ignored user config from enabling it. Other writable roots remain subject to symlink checks.
- Propagate the resolved setting through local, interactive, and exec-server sandbox paths, including `CODEX_HOME` aliases.
- Explain the opt-out in symlink rejection errors, including that it trusts targets outside `CODEX_HOME` and targets that change between commands.

## Testing

- Add macOS coverage for shell commands, patches, filesystem helpers, process execution, and interactive startup with enabled, disabled, aliased, and out-of-scope homes.

GitOrigin-RevId: 99fcdf611200c9e1b7713cf06f7fdea5bfa7f089

codex-rs/core/config.schema.json

@@ -5806,6 +5806,10 @@
       "description": "Whether the model may request a login shell for shell-based tools. Default to `true`\n\nIf `true`, the model may request a login shell (`login = true`), and omitting `login` defaults to using a login shell. If `false`, the model can never use a login shell: `login = true` requests are rejected, and omitting `login` defaults to a non-login shell.",
       "type": "boolean"
     },
+    "allow_symlinked_codex_home": {
+      "description": "Allow macOS sandbox writable roots at or beneath CODEX_HOME to traverse symlinks. Read only from the host's user config at startup; defaults to false. This grants no write access by itself, but trusts symlink targets even if they change between commands or lie outside CODEX_HOME. This setting has no effect on Linux or Windows.",
+      "type": "boolean"
+    },
     "analytics": {
       "allOf": [
         {