Run the linter only in Docker (closes #20)

script/lint now builds the new Dockerfile.lint, whose last step runs
golangci-lint. The build cache is off, so every run executes the
linter, and no image is kept. The Dockerfile lint stage calls
golangci-lint directly, since make lint is now a docker build of its
own. script/fmt no longer runs golangci-lint --fix, and
script/bootstrap no longer installs it. golangci-lint config verify is
left out: it fetches its schema over the network, unpinned. 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