fix: preserve arg0 for PTY sandbox commands without relying on PATH

[jif-oai]

Unified exec PTY sessions under Seccomp were using arg0 as the program when spawning the PTY. For the codex-linux-sandbox case, this meant we were actually executing codex-linux-sandbox via PATH lookup instead of the configured codex_linux_sandbox_exe path.

Fix: On Unix, creates a temporary directory, symlinks the real program to a filename derived from arg0 (i.e. codex-linux-sandbox), and uses that symlink as the command in CommandBuilder.

This preserves argv[0] for dispatch while always executing the configured program path, and falls back to the old behavior when program is not absolute or arg0 is unset.

[jif-oai]

@codex review

[chatgpt-codex-connector[bot]]

Codex Review: Didn't find any major issues. What shall we delve into next?

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