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

bin/gitrev.sh

@@ -1,12 +0,0 @@
-#!/usr/bin/env bash
-set -euo pipefail
-
-main() {
-    if [[ -n "${GITREV:-}" ]]; then
-        echo "$GITREV"
-    else
-        git describe --tags --always --dirty=-dirty
-    fi
-}
-
-main "$@"