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

script/generate

@@ -40,6 +40,9 @@ sha256() {
 
 main() {
     cd "$ROOT/mfer"
+    # `go install` puts protoc-gen-go in $(go env GOPATH)/bin, which is
+    # often not on PATH.
+    PATH="$PATH:$(go env GOPATH)/bin"
     require_version protoc "libprotoc $PROTOC_VERSION"
     require_version protoc-gen-go "protoc-gen-go $PROTOC_GEN_GO_VERSION"
     # Hashed before regenerating, so a missing hash tool stops the script