README: refresh for v0.1.1+v0.1.2

* Lead-in mentions 'cacher docker download --pull' alongside the
  existing 'cacher download --url' as the canonical one-liners.
* Docker section now shows the --pull cache-or-pull form first; the
  exists/upload/download dance kept for locally-built images.
* New 'Full builds.sr.ht example' section — bootstrap-cacher,
  init, install_go, cache_gomod, cache_postgres, test. Mirrors the
  shape of this repo's .builds/test.yml.
* List subcommand: --recursive and --root flags documented.
* Install: name all four published platforms; link checksums.txt.
* Releases/changelog link to bigbes.pages.srht.bigb.es/ci-cacher.
* Drop the 'directory caching is brand new' line — shipped in v0.1.0.

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.

Add BSD-2-Clause license and README

Copyright header matches the convention used in go-luarocks.
README walks through install, init/doctor, single-file and docker
caching, the new directory caching, key derivation, config
precedence, Garage compatibility, and exit codes.