fix: restore Codex environment setup table (#31337)

## Why

Creating a Codex worktree skips local environment setup because
`.codex/environments/environment.toml` omits the required `setup`
object.

## What

Restore an empty `[setup]` table with `script = ""`, preserving the
intended no-op setup while satisfying the environment parser schema.

## Validation

- Parsed the final TOML and verified `setup.script` is present as a
string.

.codex/environments/environment.toml

@@ -2,6 +2,10 @@
 version = 1
 name = "codex"
 
+# TODO(anp) make it optional to specify this field
+[setup]
+script = ""
+
 [[actions]]
 name = "Run"
 icon = "run"