fmt: format and check Markdown with prettier in a container (closes #119)

make fmt and make fmt-check now cover every Markdown file with prettier
(4-space tabs, proseWrap always), as template-app-go does: prettier is
pinned by package.json and yarn.lock and runs in a docker build on a
digest-pinned node image, never on the host. The check is forced to run
with --no-cache-filter, as script/lint is.

script/fmt-check is split into a Go half and a Markdown half because the
Dockerfile lint stage cannot run docker: that stage now runs the Go half
and script/cibuild runs the Markdown half after the build. *.md leaves
.dockerignore so documents reach the build context.

README.md, TESTING.md and TODO.md are reformatted by make fmt; apart from
the README Entrypoints entries, its make fmt line under Building and the
TODO entry, that diff is mechanical.

Model: opus-5-5

Dockerfile

@@ -1,7 +1,9 @@
 # Lint stage - fast feedback on lint issues, before the build starts.
 # The linter is invoked directly rather than through `make lint`: that
 # target shells out to `docker build -f Dockerfile.lint`, and there is
-# no docker daemon inside a docker build.
+# no docker daemon inside a docker build. For the same reason this stage
+# runs only the Go half of `make fmt-check`; script/cibuild runs the
+# markdown half after this build.
 # script/cibuild and script/docker name this stage in --no-cache-filter.
 # golangci/golangci-lint:v2.12.2 (Debian-based), 2026-08-10
 FROM golangci/golangci-lint:v2.12.2@sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240 AS lint
@@ -12,7 +14,7 @@ RUN go mod download
 
 COPY . .
 
-RUN make fmt-check
+RUN script/fmt-check-go
 RUN golangci-lint run --config .golangci.yml ./...
 
 # Build stage