Add fish support for Unix command execution shell detection

What problem are you seeing?

Codex does not appear to support fish as a Unix command execution shell. On macOS, my terminal and login environment use fish, but Codex command execution falls back to zsh. That means commands do not see the same shell-managed environment as my terminal.

This affects PATH setup, version managers, and session-based environment like 1Password/op auth state.

Thread ID: 019ddadd-5da7-7c92-8c75-c1563a6c8c4c

Feedback upload URL suggested by Codex:

https://github.com/openai/codex/issues/new?template=3-cli.yml&steps=Uploaded%20thread:%20019ddadd-5da7-7c92-8c75-c1563a6c8c4c

Expected behavior

If $SHELL or the passwd shell points to fish, Codex should either support fish command execution directly or provide an explicit config option for the default command shell.

Actual behavior

Codex detects supported Unix shells such as zsh/bash/sh, but fish is not treated as a supported shell for command execution, so the session uses a fallback shell.

Concrete ask

Add fish to the supported Unix shell detection and execution paths, likely using fish -c for non-login execution and the appropriate fish equivalent for places where Codex currently uses login-shell semantics.

Environment

• Codex CLI: 0.125.0
• Platform: macOS arm64
• Login shell: fish (/opt/homebrew/bin/fish)
• Observed command shell in Codex tool execution: zsh