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

README.md

@@ -283,9 +283,13 @@ The repo adheres to the
 standard: most Makefile targets are thin shims over an executable in
 `script/` (`build` and `clean` are the exceptions).
 
-- `script/bootstrap` installs everything needed to build and develop (git, make,
-  Go), idempotently, from nix, apt, brew or apk; it does not install the linter,
-  which only runs inside Docker.
+- `script/bootstrap` installs everything needed to build and develop,
+  idempotently: git and make from nix, apt, brew or apk, and Go, at the version
+  the `Dockerfile`'s Go image carries, from the official release archive
+  (checked against a sha256 in the script) into `~/.local/go`. `script/fmt`,
+  `script/fmt-check`, `script/precommit` and the `Makefile` put
+  `~/.local/go/bin` first on their `PATH`, so they use that Go. It does not
+  install the linter, which only runs inside Docker.
 - `script/setup` prepares a fresh clone: it runs `bootstrap`, then installs the
   git pre-commit hook.
 - `script/projectname` prints the project name; other scripts call it so they