Installer robustness: setup.sh exec-after-rm + uninstall.sh CONFIG_PREFIX guard

[ooonea]

Two small installer hygiene fixes (one per file, one commit each).

setup.sh: don't rm -rf "${tmp}" right before exec'ing it

The non-self-replacing restart branch removed ${tmp} and then
exec "${tmp}/setup.sh", so the exec always failed with ENOENT and aborted the
restart, leaving a half-finished install. Dropped the premature rm; the
EXIT/INT/TERM trap still removes ${tmp} if the exec fails. (Narrow reach:
only downloads predating the SELF_REPLACING marker take this branch.)

uninstall.sh: guard cd "${CONFIG_PREFIX}"

Under set -eu, cd "${CONFIG_PREFIX}" aborted the whole uninstall when the
config dir didn't exist (configs already removed, a re-run after a partial
uninstall, or a Merlin/custom prefix that diverges from SCRIPT_PREFIX), leaving
cake-autorate.sh/lib.sh/etc. on disk. Guarded so program-file removal still
runs.

sh -n / shellcheck -x clean (the pre-existing SC2310 infos in setup.sh are
unrelated and untouched).

[lynxthecat]

@rany2 does this look OK to merge to you? @ooonea's other PRs seemed good to me.

[rany2]

@lynxthecat It seems fine but for setup.sh I would drop self-replacing support entirely. For uninstall.sh I think it would be better to guard the entire thing with if [ -d ${CONFIG_PREFIX} ] same with every block that does a cd.

[lynxthecat]

Thoughts @ooonea?

[ooonea]

@rany2 thanks for the review — both addressed in the latest commit.

uninstall.sh — agreed, that's cleaner than my partial fix. Every cd is now guarded with [ -d … ] (both CONFIG_PREFIX and SCRIPT_PREFIX); if a prefix is absent there's nothing to remove there, so the block is skipped rather than aborting the rest of the uninstall under set -eu. I also folded the two consecutive cd "${SCRIPT_PREFIX}" blocks into one (the second cd was redundant).

setup.sh — I'd rather not rip out self-replacing wholesale in a robustness PR: the …SUPPORT_SELF_REPLACING handshake is deliberate, and removing it is a design call that's yours to make (ideally its own PR). But the only broken part is the else branch, and it's effectively dead — it fires only when the downloaded setup.sh lacks the marker, i.e. when installing a pre-marker (pre-2024) version. So instead of fixing the rm-then-exec there, I've deleted the dead else branch: a marker-less download now falls through and installs the downloaded files with the current setup.sh, rather than exec'ing an ancient one. The modern self-replacing path (marker present → exec) is untouched.

Happy to remove self-replacing entirely as a separate PR if you'd both prefer — just say the word.

[lynxthecat]

@rany2 how about now?