script/bootstrap installs the Dockerfile's Go from go.dev (closes #46)

The Gitea runner has Docker and git but no Go, and the apt path never
ran apt-get update, so on a fresh host every install failed.

Bootstrap now installs Go 1.26.8, the version in the Dockerfile's
golang image, from the release archive at go.dev, checked against a
sha256 in the script, into ~/.local/go whenever the go first on PATH
reports any other version. script/fmt, script/fmt-check,
script/precommit and the Makefile put ~/.local/go/bin first on their
PATH so they use it. The apt path runs apt-get update once, before its
first install.

Model: opus-5-5

script/fmt

@@ -4,6 +4,9 @@ set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
+# Where script/bootstrap installs Go; it cannot put it on our PATH.
+PATH="$HOME/.local/go/bin:$PATH"
+
 main() {
     cd "$ROOT"
     go fmt ./...