## Why
#23756 makes packaged Codex builds include and default to the bundled
zsh fork. The important reason to put that fork's directory at the front
of `PATH` is to keep executable-level escalation working after a command
leaves the original shell and later re-enters zsh through `env`.
The expected chain is:
1. The zsh fork runs the top-level shell command.
2. That command launches another program, such as `python3`, while
inheriting the `EXEC_WRAPPER` environment and the escalation socket fd.
3. That program spawns a shell script whose shebang is `#!/usr/bin/env
zsh` rather than `#!/bin/zsh`, and it does not close the escalation fd.
4. `/usr/bin/env` resolves `zsh` through `PATH`, so it must find the
packaged zsh fork before the system zsh.
5. Commands inside that nested script are intercepted by the zsh fork
and can still request escalation from Codex.
If `PATH` resolves `zsh` to the system shell instead, the nested script
loses zsh-fork exec interception. Commands that should request
escalation can then run only in the original sandbox, or fail there,
without Codex ever receiving the approval request.
Shell snapshots make this slightly more subtle: a snapshot can restore
an older `PATH` after the child shell starts. This PR treats the zsh
fork `PATH` prepend as an explicit environment override so snapshot
wrapping preserves it.
## What Changed
- Added shared zsh-fork runtime helpers that prepend the configured zsh
executable parent directory to `PATH` without duplicate entries.
- Applied the zsh fork `PATH` prepend to both zsh-fork `shell_command`
launches and unified-exec zsh-fork launches before sandbox command
construction.
- Kept the shell-command zsh-fork backend API narrow: it derives the
configured zsh path from session services and rebuilds its sandbox
environment from `req.env`, rather than accepting a second, competing
environment map or a separately threaded bin dir.
- Kept Unix-only zsh-fork `PATH` mutation out of Windows clippy-visible
mutability.
- Added coverage for duplicate `PATH` entries, for preserving the zsh
fork prepend through shell snapshot wrapping, and for the nested
`python3` -> `#!/usr/bin/env zsh` escalation flow.
## Testing
- `just fmt`
- `just fix -p codex-core`
I left final test validation to CI after the latest review-comment
cleanup. Before that cleanup, `just test -p codex-core zsh_fork` passed
locally for the zsh-fork-focused tests.
Michael Bolin
bc10e5b390