install.sh: verify checksums without coreutils extensions; bump to 0.2.1

sha256sum -c --ignore-missing is a coreutils extension. Alpine ships
busybox sha256sum, which prints its usage and exits non-zero, so the
bootstrap died with a 'checksum mismatch' that was nothing of the sort —
on the exact distro the sr-ht-cover manifest builds on. Pull the expected
hash out of checksums.txt with awk and compare strings instead; busybox,
coreutils and macOS shasum all agree on 'sha256sum FILE'.

Also stage the binary inside the target directory and rename it into
place, so an install over a running cacher can't hit ETXTBSY.

Verified against the published release on alpine:edge (busybox) and
ubuntu:noble (coreutils), and across all four published assets by faking
uname: Linux/x86_64, Linux/aarch64, Darwin/x86_64, Darwin/arm64 each
fetch the matching binary and verify.

Bump VERSION to 0.2.0 for tag

Bump VERSION to 0.1.2; widen landing page to 55rem

Bump VERSION to 0.1.1 for tag

Bump VERSION to 0.1.0 for tag

Initial cacher v0.0.1-dev — S3-backed CI cache helper

Replaces tarantool-protobuf/.builds/lib/ci-lib.sh (and its
two shell helpers s3_cache_or_curl + s3_cache_docker_image)
with a single static Go binary.

Commands:
  init / doctor          persist config + smoke-test credentials
  download / upload      single-file cache with URL fallback + sha256
  exists / list / delete cache management
  key                    resolve key template (for shell scripting)
  docker {exists,download,upload}  streamed save/load via zstd
  dir {download,upload}            tar+zstd directory caching (new)

Key derivation via --hash-from <path> (repeatable; file or dir),
matching the existing sha256sum | cut -c1-16 shell convention.
Garage-compatible S3 client (path-style, checksums when_required).