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

script/lint now builds only the lint stage of the main Dockerfile
(docker build --no-cache --target lint), whose build runs the linter, so
a successful build is a clean lint. It is uncached because a cached
build runs no linter, and a trap removes the image it tagged; the tag
carries the process ID so concurrent runs do not collide. The 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.

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 this stage, 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