Move Alpine's repository setup into the guest prelude

[NovusEdge]

Problem

Four bundled recipes carry the same Alpine repository setup. devtools,
build-deps, service-tools, and pkg-tools each open install-alpine.sh
with the same loop:

n=0
until setup-apkrepos -c -1; do
    n=$((n + 1))
    [ "$n" -ge 30 ] && { echo "apk database stayed locked; giving up" >&2; exit 1; }
    sleep 2
done

Each copy carries a different comment, and each one skips stoat_pkg_setup.
The Global Constraints for a bundled recipe say every script calls
stoat_pkg_setup before it installs, so all four scripts state an exception.

Cause

Alpine's guest prelude defines stoat_pkg_setup as apk update. That command
does not enable the community repository, and it does not wait for the apk
database lock. setup-apkrepos -c -1 does both. No recipe wants what the
prelude offers, so every recipe replaced it.

Proposal

Move the loop into Alpine's stoat_pkg_setup in
internal/guest/bundled/alpine.toml. The four scripts then call
stoat_pkg_setup like the scripts for every other guest, the exception
disappears, and the retry behaviour has one definition.

Acceptance criteria

• internal/guest/bundled/alpine.toml performs the mirror selection, the
  community repository, the index refresh, and the lock retry.
• The four install-alpine.sh scripts call stoat_pkg_setup and hold no
  repository logic.
• A test covers the retry: a locked database returns non-zero, and the setup
  verb tries again.
• The Alpine row of the recipe live matrix passes with the recipes selected
  together.

Evidence

Found by the whole-branch review of #100. The duplication is byte-identical
apart from the comments.