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:-}.

docs/RELEASE_PROCESS.md

@@ -37,6 +37,7 @@ Use this checklist for every release:
 - [ ] Confirm install docs/website mention **AppImage** when this release ships AppImages
 
 ### Testing & Verification
+Install `just` from https://just.systems or the distro package `just`.
 - [ ] Run `just test` - All tests pass
 - [ ] Run `just lint` - No linting errors
 - [ ] Run `just typecheck` - No type errors
@@ -258,7 +259,8 @@ Also align `web/package-lock.json` top-level `version` fields with `web/package.
 
 ### Step 5: Verify Everything Works
 
-Run the full verification suite:
+Run the full verification suite. Install `just` from
+https://just.systems or the distro package `just`.
 
 ```bash
 # Run tests