Makefile: thin shims only, add make build (closes #73)

The Makefile is now one shim per script/ entrypoint, in the order of
the canonical model Makefile, plus build, generate and fuzz. The new
script/build writes bin/mfer with the urfave_cli_no_docs tag and the
git describe revision that script/docker stamps. script/generate now
adds the Go bin directory to PATH itself, and the Docker lint stage
calls script/gofumpt --check directly. Removed: the tarball-caching
rules and their ignore entries, godoc, ci, fixme, clean, run, default,
fmt-check-go, fmt-check-md, and bin/gitrev.sh, which only the
Makefile called.

Model: opus-5-5

Dockerfile

@@ -11,7 +11,7 @@ COPY . .
 # Go half of fmt-check only: this image has no node, so no prettier. The
 # markdown half runs in the mdfmt stage below. The image has no gofumpt
 # either; script/gofumpt builds the version it pins with `go run`.
-RUN make fmt-check-go
+RUN script/gofumpt --check
 # 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 ./...