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/gofumpt

@@ -4,8 +4,8 @@
 # Takes exactly one mode argument, --write or --check, and runs the same
 # gofumpt version over the same files in both modes. script/fmt and
 # script/fmt-check both go through here, and so does the Docker lint
-# stage (make fmt-check-go), so what gets formatted and what gets
-# verified cannot drift apart.
+# stage, so what gets formatted and what gets verified cannot drift
+# apart.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"