ci: date the nightly release, so a downstream can pin one

[lens0021]

The problem

The nightly says what it is for:

Build the per-platform tarballs from the latest main into a rolling "nightly" prerelease, so downstreams (e.g. wikven) can track SifterSearch without a formal release.

It then published to a single rolling nightly tag, deleted and recreated at main's tip each night — which is the one shape that downstream cannot use.

wikven pins SifterSearch by release tag in its Dockerfile, and says of the pin sitting directly beneath it:

Commits, not the branch tip: REL1_46 takes translatewiki updates weekly, so a branch pin builds a different Translate from the same wikven commit a week later.

It also now asserts in CI that two bakes of the same source are byte-identical. A tag whose contents change nightly is a branch pin wearing a tag's clothes, so the nightly could not be pinned by the very project it was built for — only a formal release could, which is what the nightly was meant to spare them.

The change

nightly-<date> names one build forever. A downstream can pin it, stay reproducible, and updatecli has something to bump.

Nothing references the rolling tag — not this repository, not wikven — so it goes rather than staying on as an alias whose whole behaviour is the problem.

Two smaller faults went with the old shape, both from publishing before the assets existed:

• delete-then-recreate left a window where the tag resolved to nothing and a downstream fetch would 404;
• the release was created published, then packaged, so it was visible carrying no tarballs for the length of a five-platform build.

Creating the release as a draft, attaching every platform, then un-drafting closes both — and is what GitHub's immutable releases require in any case, since assets lock the moment a release is published and only a draft can still be written to.

A feat/fix gate comes with it, so a day of chores or CI-only commits does not mint a release identical to yesterday's.

This is wikven's own nightly.yml, structure and reasoning alike; the two repositories were solving the same problem two different ways, and only one of them had written down why.

What a downstream does with it

wikven's Dockerfile takes a tag directly, so tracking a nightly needs no change there:

ARG SIFTERSEARCH_VERSION=nightly-2026-08-18

and reproducibility survives, because that tag will hold the same tarballs next month.

Notes

• No change to what is built or packaged — package.yaml is untouched apart from its header comment, and still receives a tag and uploads the same assets to it.
• The existing nightly release and tag are left in place by this PR; they can be deleted whenever you like, since nothing consumes them.
• The gate looks for the last nightly-* tag, and the current tag is nightly with no dash, so the first run after this merges is unconditional — which is what you want for the first dated build.
• Independent of fix: index each page in its own language, not the wiki's #69; branched from main, no overlap.

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