Run the linter only in Docker (closes #20)

script/lint now builds only the lint stage of the Dockerfile, without
the build cache, so every run executes the linter; the image is tagged
simplelog-lint. The lint stage calls golangci-lint directly, since make
lint is itself a docker build of that stage. script/cibuild and
script/docker also build without the cache, so their check steps always
run. script/fmt no longer runs golangci-lint --fix, and script/bootstrap
no longer installs it. golangci-lint config verify is left out, on the
owner's ruling. The README, TODO.md and the script/cibuild comment say
what now runs.

Model: opus-5-5

script/bootstrap

@@ -54,12 +54,7 @@ main() {
     if missing git; then pkg_install git git git git; fi
 
     if missing go; then pkg_install go golang go go; fi
-    # golangci-lint is packaged in nix, brew, and apk; there is no apt
-    # package (on apt hosts, install it from a hash-verified GitHub
-    # release archive manually, never curl | sh).
-    if missing golangci-lint; then
-        pkg_install golangci-lint golangci-lint golangci-lint golangci-lint
-    fi
+    # golangci-lint is not installed: it runs only in docker (script/lint).
 
     go mod download