fix(install): leftover review notes from #700 and #705 (#706)

* fix(install): leftover review notes from #700 and #705

The scratch dir now holds pip logs and model download temps, so a failed
run has something to inspect. Dep, venv, leftover-process, and pip
failures use exit codes 2/3/4 instead of a blanket 1. --help says the
transcript is written during installation, and the tag hint is
--tag=<release>.

AGENTS.md no longer claims the hash-pinned exports are consumed (they
wait for phases 2/3/5). Docs add a just install line and
vocalinux[vosk] next to the PyPI commands. Settings points at
--engine=whisper. The whisper.cpp fallback installs ".[vosk]".

Fixes leftover review notes from #700 and #705.
Refs #701.

* fix(install): do not let pip build PyGObject from sdist

Fresh --auto --skip-system-deps died on pygobject-3.56.3 even with
distro python3-gi present. Install the other runtime deps from
pyproject.toml, then pip install --no-deps. Extra installs use the
extra's packages only, not pip install ".[extra]".

Fixes leftover review notes from #700 and #705.
Refs #701.

* fix(install): tolerate unset XDG_SESSION_TYPE under set -u

--auto with DISPLAY but no login session died on
XDG_SESSION_TYPE: unbound variable. The session probe now uses
${XDG_SESSION_TYPE:-}, ${WAYLAND_DISPLAY:-}, and ${DISPLAY:-}.

src/vocalinux/ui/settings_dialog.py

@@ -5005,7 +5005,7 @@ class SettingsDialog(Gtk.Dialog):
 Installation Options:
 
 1. Using the installation script:
-   ./install.sh --with-whisper
+   ./install.sh --engine=whisper
 
 2. Manual installation in virtual environment:
    source venv/bin/activate