Lint static/js/ with ESLint in Docker (closes #120)

`REPO_POLICIES.md` binds the repo to a JavaScript styleguide, but nothing checked `static/js/`. ESLint, pinned by `package.json` and `yarn.lock`, now lints it with the styleguide's two checkable rules, `no-var` and `prefer-const`. It runs only in Docker on a digest-pinned node image: a `js-deps` stage installs ESLint and stays cached until the manifests change, and a `js-lint` stage runs it. `script/lint` builds `js-lint`, so `make lint` and `make check` fail on a violation, and the image build depends on it as on the repo's other checks. ESLint, node and yarn are not prerequisites, and `make lint` never uses a host copy.

Model: opus-5-5

.gitignore

@@ -15,6 +15,9 @@ bin/
 # Go vendor directory
 vendor/
 
+# ESLint and its dependencies, installed from yarn.lock
+node_modules/
+
 # IDE specific files
 .idea/
 *.swp