runtime: prepend zsh fork bin dir to PATH

[bolinfest]

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.