ci: the family's build pipeline, and PKGVER by export

Brings this manifest to the shape the sibling services share.

cacher is installed by install.sh instead of a raw curl of the binary, in
two tasks: install.sh appends its PATH export to ~/.buildenv, which only
the next task sources. The old fetch had no -f, so an HTTP error body was
written to ~/.local/bin/cacher and chmod +x'd; the installer verifies
against checksums.txt.

The version task drops the sed of the tracked APKBUILD. Measured on
go1.26.5, rewriting a tracked file sets vcs.modified=true and Go stamps
+dirty into Main.Version for the whole abuild run, so PKGVER is exported
and APKBUILD reads pkgver="${PKGVER:-0.0.0}". One git describe decides
it: the tag, the tag plus commits, or the commit count. Tags join
allow-refs so pushing v0.9.0 produces the 0.9.0 apk. The export alone is
not enough — abuild works inside the checkout and puts src/, pkg/ and
GOTMPDIR=tmp/ there, so .gitignore covers all three.

cache_restore takes --optional rather than || true, which also swallowed
bad credentials and a dead bucket, and gains the half-restored module
cache repair block. cache_save moves after publish so an S3 hiccup cannot
strand a good apk, and loses its cacher exists guards: dir upload already
skips a present key. publish gains the no-credentials gate.

The stylesheet is now built in CI: sassc and minify, the scss task that
materializes the shared partials no apk ships, make css before the
compile and make check-css after it. The committed
web/static/main.min.79713f25.css stays tracked for now — build() prints
git status and the digest so the first CI run says whether the pipeline
reproduces those bytes, and dropping it from the index is a second step.

package() calls make install-files. abuild runs it in a fresh process
that never called build(), so make install would relink the binary from a
cold cache and ship a compilation nothing had checked.

Reasoning lives in docs/ci.md: the manifest is a varchar(16384) and a
branch over the cap gets no CI at all