fix(executor): restore tty foreground pgrp for interactive sessions

[Pierozi]

Summary

• PR feat(executor): forward signals to sandboxed process group to prevent orphans #39 placed the sandboxed child in its own process group for signal forwarding, but this detached it from the tty's foreground group — any tty read then raises SIGTTIN and the interactive shell hangs indefinitely.
• Adds TtyForegroundGuard: after spawning, hands the tty foreground to the child's pgrp via tcsetpgrp (with SIGTTOU suppressed during the handoff), and restores the original foreground on drop.
• RAII pattern mirrors the existing PgidKillGuard, ensuring the parent reclaims the tty on all exit paths including panics.

Test plan

• Run sx online in an interactive terminal — shell should accept input without hanging
• Run sx offline and type a command — verify normal interactive behavior
• Ctrl-C inside the sandbox exits cleanly (signal forwarding from feat(executor): forward signals to sandboxed process group to prevent orphans #39 still works)
• Run sx -- echo hi (non-interactive) — guard is a no-op, output is correct
• Run cargo test — all existing tests pass