Run all linting in Docker via Dockerfile.lint (closes #90)

script/lint now only builds the new Dockerfile.lint, which copies the
repo into the pinned golangci-lint image and runs the linter as a build
step, so a successful build is a clean lint. It builds with --no-cache,
because a cached build runs no linter, and removes the image it tagged
when it exits. The main Dockerfile lint stage calls golangci-lint
directly, since make lint now needs Docker. Nothing installs or runs
golangci-lint on the host any more: bootstrap and the Makefile drop the
install, and script/fmt drops golangci-lint run --fix. The gofmt check
script/lint repeated stays in script/fmt-check.

Model: opus-5-5

Dockerfile

@@ -14,7 +14,9 @@ RUN touch mfer/mf.pb.go
 # Go half of fmt-check only: this image has no node, so no prettier. The
 # markdown half runs in the mdfmt stage below.
 RUN make fmt-check-go
-RUN make lint
+# The linter directly, not `make lint`: script/lint builds Dockerfile.lint,
+# and there is no docker inside this build.
+RUN golangci-lint run --config .golangci.yml ./...
 
 # Markdown/JSON format stage — prettier needs node, which the Go images
 # do not have. node:22.17.0-bookworm-slim (2026-08-09); ships node