ci: export the version instead of sed-ing the APKBUILD

The version task rewrote pkgver= in the tracked APKBUILD before abuild ran.
Go decides the vcs.modified half of the build stamp it records in every
binary from git status --porcelain, so that one sed made every binary in
the apk call itself <sha>-dirty for the life of the package. Measured on
go1.26.5. PKGVER now reaches abuild through ~/.buildenv and the APKBUILD
reads pkgver=${PKGVER:-0.0.0}; a local abuild builds an honest 0.0.0.

One git describe now decides it — a tag, else tag_git<n>, else the family's
0.0.<commit count> — and refs/tags/v* joins allow-refs so a tag builds.

The export alone is not enough: an untracked file at go build time sets the
same flag, and abuild works in this checkout. .gitignore takes /src/, /pkg/
and /tmp/, the last being the one abuild points GOTMPDIR at while
MAKEFLAGS=-j runs three go builds that can see each other's work dirs.

Around it, the shape the siblings converged on: cacher installed through
install.sh (checksum-verified, and the raw curl it replaces had no -f, so a
404 body was chmod +x-ed) in two tasks, since its PATH export lands in
~/.buildenv and only the next task sources it; scss through cacher --exec,
whose seed upload is best-effort, instead of an if-block that made an S3
hiccup fatal and a genuine S3 error indistinguishable from a miss;
--optional and the module-cache repair block in cache_restore; cache_save
after publish, without the cacher exists guards an upload already implies;
and publish gated on a readable ~/.apk-ci.env so a secretless submission
reports what it built instead of failing.

go mod download and NOT go mod download all: all resolves dependencies'
test dependencies and appends their hashes to go.sum, which is a tracked
file and therefore the very -dirty stamp this commit removes. On this tree
it added 426 lines, 671 -> 1097; plain download leaves go.sum untouched and
go mod verify still passes.

APKBUILD gains GOFLAGS=-trimpath -modcacherw, without which the binaries
embed builder paths and the module cache is packed read-only into the cache
tarball that the next build then cannot unpack. package() calls the new
install-files, which does not build: abuild runs it in a fresh process that
never called build(), so make install recompiled all three binaries from a
cold cache and shipped a copy nothing had checked. The bare ls of the
stylesheet becomes make check-css, which counts — web/router.go takes the
first glob match, so two stylesheets are as wrong as none. The three binary
targets lose their if [ -d ./cmd/... ] guards, which now only serve to turn
a deleted cmd/ into a green build that packages nothing.

The manifest is a varchar(16384) and over the cap a branch gets no CI at
all, so the reasoning moved to docs/ci.md and the tasks carry pointers.