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.