Detect the PTY foreground process without shell integration - #723

Ghostel could only tell that "a command is running" through OSC 133
shell-integration markers, and never which program.  Query the PTY's
foreground process group instead — the same signal terminal emulators
use for close confirmation:

- PosixPtyProcess gains a TIOCGPGRP query on the PTY primary, exposed
  through the NativeProcess facade (under the backend-handoff mutex)
  and registered as ghostel--pty-foreground-pgid.  The ioctl must run
  on the primary: a re-opened replica fd is not the caller's
  controlling terminal, so the kernel rejects it with ENOTTY.  ConPTY
  has no foreground process-group concept and returns nil.
- ghostel-foreground-pid dispatches per PTY path: the native module
  owns the primary fd on the native path; Emacs owns it on the
  Emacs-PTY path, where process-running-child-p answers the same
  question (nil = shell at prompt, integer = child pgid, t = unknown).
- ghostel-command-running-p combines the probe with the OSC 133 C→D
  state; the OSC arm still covers remote (TRAMP) sessions, where the
  probe only sees the local client.
- ghostel-query-before-killing's auto setting now defers to
  ghostel-command-running-p, so it works without shell integration and
  also asks while an ssh client or nested shell holds the foreground —
  killing the buffer would kill that session.
- ghostel--shell-pgid supplies the baseline: ghostel-macos-login-shell
  runs the shell under login(1), which forks, so ghostel--pid is login
  and the idle shell never matches it.  Only with such a wrapper
  (ghostel--shell-forked) is a direct child of ghostel--pid adopted as
  the shell and cached; without one, a command the shell starts is a
  direct child too and would be adopted in its place.
- New abnormal hook ghostel-foreground-change-functions (buffer, pgid,
  comm), sampled after each redraw while non-nil and run outside the
  render inhibits.  The foreground identity is the (pgid . comm) pair:
  an early sample can catch a forked command before exec, still under
  the shell's name, and the pair comparison re-fires once the real
  name shows up with the program's next output.  process-attributes is
  slow, so comm is re-read only when the pgid changed or while a
  command still carries the shell's comm (remembered from samples where
  the shell holds the foreground); an idle prompt or an exec'ed command
  costs one ioctl per redraw.

- ghostel-bookmark--shell-idle-p and consult-ghostel--history-insert
  move off the raw OSC 133 flag onto ghostel-command-running-p, so they
  also refuse while an ssh client or nested shell holds the terminal,
  with no shell integration required.

The predicate needs a shell to compare the foreground against, so it
never fires in a buffer that runs a program directly (ghostel-exec,
ghostel-compile); the defcustom, CHANGELOG and README say so.

Tests cover both PTY backends live (probe tracking, hook firing
through the redraw path, interrupt/reclaim, and a forked shell reading
as idle) plus the predicate, child-state mapping, shell-pgid adoption,
and the comm re-read gating with mocked probes.