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

[moukrea]

Summary

Fixes remaining issues from the hook rework (#14):

• Drop tee-based capture: exec > >(tee -a FILE) 2>&1 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. Only snag output loses scrollback for hooked sessions.

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