## Why
When unified exec is configured to launch through the zsh fork, local
commands should not let the model override the shell binary with the
`shell` parameter. The configured zsh fork is the mechanism that makes
`execv(2)` interception reliable, so exposing `shell` for local zsh-fork
execution would create a confusing API surface and undermine the
composition.
Remote environments are different: zsh-fork interception is local-only,
so remote unified-exec calls must keep direct unified-exec behavior and
still expose `shell` when a remote environment can be selected.
## What Changed
- Taught the `exec_command` schema builder to omit the `shell` parameter
when requested.
- Hid `shell` from the unified-exec tool schema only when zsh-fork
unified exec applies to all selectable environments.
- Kept `shell` visible when any remote environment can be targeted,
because those calls run through direct unified exec.
- Made unified exec choose the effective shell mode per selected
environment: local environments keep zsh-fork mode, remote environments
use direct mode.
- Left direct unified-exec behavior unchanged, including support for
model-specified shells there.
## Verification
- Added schema coverage showing `exec_command` can hide `shell`.
- Added planner coverage showing zsh-fork unified exec hides `shell` for
local-only execution while direct unified exec still exposes it.
- Added planner coverage showing `shell` remains visible when a remote
environment is available.
- Added handler coverage showing remote environments use direct
unified-exec shell mode instead of zsh-fork mode.
- Ran the focused `codex-core` shell-parameter and zsh-fork tests.
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/24980).
* #24982
* #24981
* __->__ #24980
Michael Bolin
feb9eddc51