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

README.md

@@ -29,7 +29,7 @@ is committed, so no `protoc` toolchain is required:
 ```sh
 git clone https://git.eeqj.de/sneak/mfer.git
 cd mfer
-go build -o bin/mfer ./cmd/mfer
+make build
 ```
 
 Generate a manifest for a directory tree, verify it later, and fetch a published
@@ -76,6 +76,9 @@ provide:
   such as `script/docker`
 - `script/test` — run the test suite (`go test`); one test fails when
   `mfer/mf.proto` no longer matches the hash `script/generate` recorded
+- `script/build` (`make build`) — build the `mfer` binary into `bin/mfer`,
+  stamped with the revision `mfer version` prints: the output of
+  `git describe --tags --always --dirty`, as `script/docker` passes it
 - `script/generate` (`make generate`) — regenerate `mfer/mf.pb.go` from
   `mfer/mf.proto` and record the hash of that `mfer/mf.proto` in
   `mfer/mf.proto.sha256`; the only thing that regenerates the committed
@@ -85,8 +88,8 @@ provide:
   [its release](https://github.com/protocolbuffers/protobuf/releases/tag/v33.4)
   and put its `bin/protoc` on `PATH`) and `protoc-gen-go` v1.36.11
   (`go install google.golang.org/protobuf/cmd/protoc-gen-go@v1.36.11`, which
-  installs it in `$(go env GOPATH)/bin`; `make generate` adds that directory to
-  `PATH`)
+  installs it in `$(go env GOPATH)/bin`; `script/generate` adds that directory
+  to `PATH`)
 - `script/fuzz` — fuzz the manifest parser for one minute; run by hand
   (`make fuzz`), never by CI, while `script/test` runs its committed seed corpus
   as ordinary tests
@@ -98,8 +101,8 @@ provide:
 - `script/gofumpt` — run `gofumpt` over every Go file in the repository in the
   given mode, `--write` or `--check`, at the one version it pins (built on
   demand by `go run`, so nothing installs it); `script/fmt`, `script/fmt-check`
-  and the Docker lint stage (`make fmt-check-go`) all go through it, so they
-  cannot disagree about Go formatting
+  and the Docker lint stage all go through it, so they cannot disagree about Go
+  formatting
 - `script/prettier` — run prettier over the repository's canonical file set
   (Markdown and JSON, minus `.prettierignore`) in the given mode, `--write` or
   `--check`; the single definition of that file set, so `script/fmt` and