publish.yml: restore artifacts block now that storage works

Job 148 reached the upload stage where 30347e5 didn't — admin has
configured the artifact storage backend, so the four cacher binaries
+ checksums.txt are useful again as a 90-day-TTL view on the job page
alongside the permanent pages.sr.ht copy.

publish.yml: dogfood cacher for deps, drop failing artifacts block

Two changes:

* Drop `artifacts:` block. The srht.bigb.es self-hosted instance
  doesn't have artifact storage configured (the upload step fails
  silently right after "Running task stage_artifacts"). Pages.sr.ht
  already serves the binaries with stable URLs, so artifacts were
  just redundant. Removes the stage_artifacts task too.

* Cache the Go and goreleaser tarballs through cacher itself.
  Bootstrap: curl the previously-published cacher binary from pages;
  cacher init against s3.bigb.es using the existing srht-registered
  secrets (same UUIDs as tarantool-protobuf .builds); then cacher
  download "goreleaser/<ver>.tar.gz" --url "https://github…" handles
  cache-or-fetch + back-fill. First run after this commit pays the
  full curl cost and seeds s3; every later run is an S3 hit.

  hut isn't cached — no upstream binary release, building from
  source is small + fast.

publish.yml: override hut pages origin to public URL

The srht.bigb.es OAuth worker writes the internal docker-compose
hostname (http://pages:5112) into ~/.config/hut/config, which is
unreachable from outside the cluster. sed the config to use the
public origin (https://pages.srht.bigb.es) before publish.

This is a srht admin-side gotcha; only `builds` and `meta` get
public origins in the worker config, the other services
(pages/git/hub/todo/paste) all keep internal hostnames.

publish.yml: goreleaser --skip=validate

When a CI manifest patch lands after tagging, the version tag won't
point at HEAD. --skip=validate lets goreleaser proceed with the
already-tagged version anyway. Safe because the binary built from
HEAD vs the tag is byte-identical (only publish.yml differs).

publish.yml: mkdir ~/.local/bin before extracting goreleaser

Caught by job 143 install_goreleaser step — install_go creates
~/.local but not ~/.local/bin, so the tar -C target didn't exist.

publish.yml: hut version (not --version)

Caught by job 141 install_hut step. `hut` doesn't expose a --version
flag; the subcommand is `hut version`. `go install` had succeeded
silently and the failure was just the version probe.

Add goreleaser + CHANGELOG.md; cross-platform downloads on site

* .goreleaser.yml — builds linux/{amd64,arm64} and darwin/{amd64,arm64}
  as raw binaries (formats: binary) with CGO_ENABLED=0, trimpath, version
  ldflag injection. GitHub release disabled; we publish via pages.sr.ht.

* CHANGELOG.md — Keep-a-Changelog format. v0.1.0 entry summarizes the
  full surface (init/doctor, single-file/docker/dir caching, --hash-from,
  Garage compat, e2e suite, CI manifests).

* .builds/publish.yml — installs goreleaser binary release (pinned to
  v2.7.0), runs `goreleaser release --clean --skip=publish`, flattens
  the 4 dist/cacher_<os>_<arch>_v<n>/cacher paths into
  pages/cacher-<os>-<arch>, regenerates checksums.txt to match the
  published filenames, renders CHANGELOG.md to HTML via cmark with
  heading demotion (h1→drop, h2→h3, h3→h4), substitutes per-platform
  sha256s into docs/index.html, and ships everything to
  bigbes.pages.srht.bigb.es/ci-cacher/ plus the build artifacts list.

* docs/index.html — Download section is now a 4-row table with per-
  platform sha256s, plus a Changelog section that embeds the rendered
  CHANGELOG.md inline.

Add docs/index.html landing page; publish.yml substitutes build info

Replaces the inline 4-line index.html written in publish.yml with a
proper text-mostly landing page under docs/index.html — sections for
what it is, install, the loop it replaces, commands, why, and a
'this build' block populated at publish time from VERSION, SHA-256,
and build timestamp via sed.

Single-column, ~44rem max width, system fonts, monospace for code,
dark-mode via prefers-color-scheme. No JS, no images, no remote
assets. ~130 lines of HTML+CSS.

Fix pages.sr.ht target: bigbes.pages.srht.bigb.es/ci-cacher

cacher.srht.bigb.es was a guess and isn't configured. Use the
user-level pages domain (bigbes.pages.srht.bigb.es) with hut's
`-s /ci-cacher` subpath flag so this manifest only touches files
under that prefix — leaves room for other projects under the same
domain without clobbering.

Add builds.sr.ht CI: unit, e2e, and tag-only publish

Three independent manifests under .builds/ (within the 4-build-per-push
cap):

* unit.yml — ubuntu/noble, go test ./..., smoke ./cacher version. Runs
  on master and tags.

* e2e.yml — ubuntu/noble + docker.io, runs the testcontainers e2e suite
  (go test -tags=e2e). Pulls dxflrs/garage:v2.3.0 per Garage container.
  Runs on master and tags.

* publish.yml — tag-only. Builds linux-amd64 static binary, ships it
  two ways: as a build artifact (visible on the job page, 90-day TTL)
  and via hut pages publish to cacher.srht.bigb.es with a minimal
  index.html and embedded sha256 for verification. Uses
  oauth: pages.sr.ht/PAGES:RW so no manual secret registration needed.

Go is installed from upstream tarball in every manifest because
ubuntu/noble's golang-go is too old for the go.mod requirement
(1.26.3). hut is built from source in publish.yml since it's not
packaged for noble.

README install snippet updated to point at the pages URL.