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

TODO.md

@@ -24,6 +24,9 @@ files it depends on: .golangci.yml, REPO_POLICIES.md, .editorconfig,
 
 # Completed Steps
 
+* 2026-10-06: the linter runs only in Docker: `script/lint` builds
+    `Dockerfile.lint` without the build cache, and `script/bootstrap` no
+    longer installs golangci-lint
 * 2026-08-10: fixed every handler discarding slog attributes: console,
     JSON and webhook handlers now emit record attributes, accumulate
     WithAttrs without mutating the receiver, and honour WithGroup;