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/build

@@ -0,0 +1,20 @@
+#!/bin/sh
+# script/build: build the mfer binary into bin/mfer, stamped with the
+# revision `mfer version` prints, derived as script/docker derives it.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    # Own line: a failing command substitution inside an argument does
+    # not trip `set -e`, so the inline form degrades silently to an
+    # empty constant.
+    version="$(git describe --tags --always --dirty 2>/dev/null || true)"
+    [ -n "$version" ] || version="unknown"
+    go build -tags urfave_cli_no_docs \
+        -ldflags "-X main.Gitrev=$version" \
+        -o bin/mfer ./cmd/mfer
+}
+
+main "$@"