Install Hugo at a deliberate, hash-verified version (closes #26)

script/bootstrap did `pkg_install hugo hugo hugo hugo`, so the tool that
produces the published artifact was whatever the base image's package
repo happened to serve: alpine 3.21 gives hugo 0.139.0, about two years
behind upstream, chosen by nobody, and liable to change silently on any
base image digest bump. Hugo's version is a property of the site's
output, not of the build environment, so it now gets pinned like every
other external reference in this repo.

It is installed with `go install github.com/gohugoio/hugo@v0.164.0`,
which verifies the module against the sum.golang.org checksum database.
That is genuine hash verification rather than bare version pinning, it
is the mechanism REPO_POLICIES.md already names for Go, and it needs no
hand-maintained sha256. It also keeps a single pinned base image: a
digest-pinned Hugo container would have reintroduced the second base
image that #7 deliberately removed.

Two constants carry the decision, each with the canonical
`# name version, YYYY-MM-DD` comment:

  - HUGO_VERSION=v0.164.0, the current stable release.
  - HUGO_GOTOOLCHAIN=go1.26.5. hugo v0.164.0's go.mod requires
    go >= 1.26.0 and alpine 3.21's go package is 1.23.9 built with
    GOTOOLCHAIN=local, so a bare `go install` refuses to run at all.
    Naming the toolchain makes Go fetch it through the module proxy and
    verify it against sum.golang.org like any other module, so the chain
    stays hash-verified end to end and the compiler is deliberate too.

CGO_ENABLED=0 is deliberate: standard Hugo, not extended. Verified that
this site uses nothing extended provides - no .scss/.sass, no
resources.ToCSS, no PostCSS, and no image processing; the CSS is plain
and inlined by readFile in baseof.html. The `+extended` on the apk build
this replaces was incidental, and the script says so, so a later change
does not assume extended is required.

The binary is placed in /usr/local/bin rather than left in a GOPATH bin
directory, because it has to be on the default PATH of a *fresh* shell:
the Dockerfile's `RUN make check` and deploy.yml's `script/test` step
each start their own shell. The location is overridable via
HUGO_BIN_DIR for unprivileged installs, and `go install` itself runs as
the invoking user so a workstation's module cache is not populated as
root.

The idempotency guard is version-aware instead of `missing hugo`: an
older hugo already on PATH must be replaced, not accepted, or the pin
means nothing. A same-version build that happens to be `+extended` is
accepted, since it renders this site identically. After installing, the
script re-checks what `hugo` on PATH actually resolves to and fails
loudly if something else shadows it.

Rendered output was compared three ways in a container carrying both
binaries - apk 0.139.0 against 0.164.0 on identical sources. Across the
whole public/ tree the only byte that differs is the generator meta
tag's version string, which is the change describing itself. The RSS
<language> element and the html lang attribute are unchanged.

Cold `script/cibuild` is 2m36s, within the five-minute budget: 52.6s of
it is the bootstrap layer (apk go, toolchain fetch, compile) and 100s is
image export. The check image grows to 683 MB because the Go toolchain
and module cache stay in the bootstrap layer; that image is only ever
built to run checks, never published or deployed.

README.md

@@ -46,8 +46,10 @@ standard: normalized scripts in `script/` are the entrypoints for the
 development workflow, and the Makefile targets are thin shims that call them. We
 provide:
 
-- `script/bootstrap` — install all build dependencies (git, make, hugo,
-  node/npm) idempotently
+- `script/bootstrap` — install all build dependencies (git, make, go, hugo,
+  node/npm) idempotently. Hugo is pinned to an exact version and installed with
+  `go install`, which verifies it against `sum.golang.org`; the version is the
+  `HUGO_VERSION` constant at the top of the script
 - `script/setup` — prepare a fresh clone: run `script/bootstrap` and install the
   git pre-commit hook
 - `script/test` — the correctness check: a clean `hugo --minify` production