fix(hook): drop tee capture and use tcgetpgrp for fg process

[moukrea]

Summary

• Drop tee-based capture from hook: exec > >(tee ...) made stdout a pipe, breaking isatty() — TUI apps (nano, vim) couldn't initialize ncurses properly and keyboard shortcuts failed. The hook now only exports SNAG_SESSION with zero side effects. snag ls/cwd/ps/attach/send all work via /proc and the stolen master fd.

• Use tcgetpgrp for FG process: replaces the /proc scan heuristic (which picked up background tee as the FG process) with nix::unistd::tcgetpgrp(master_fd) — the kernel's authoritative answer for the foreground process group. Shows idle at prompt, actual command name when running something.