Pin the remaining developer tool installs (closes #68)

gofumpt and protoc-gen-go are tools of a separate Go module in bin/tools,
so `go tool` builds them from source checked against bin/tools/go.sum and
mfer's own module gains no dependencies. script/bootstrap downloads that
module, and unpacks protoc 33.4 into bin/protoc from its release archive
after checking the sha256 it holds for the platform. script/generate runs
that protoc with the pinned plugin, so mfer/mf.go and its go:generate line
go. script/prettier runs only the node_modules prettier, fails when it
differs from the package.json pin, finds the node bootstrap installed
through nvm by the version in .nvmrc, and runs prettier once. Comment and
package.json fixes.

Model: opus-5-5

script/gofumpt

@@ -10,10 +10,10 @@ set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# gofumpt v0.12.0, 2026-10-04. `go run` fetches and builds exactly this
-# version, so neither a developer machine nor the lint image needs
-# gofumpt installed.
-GOFUMPT="mvdan.cc/gofumpt@v0.12.0"
+# The gofumpt version is the one bin/tools/go.mod pins. `go tool` run in
+# bin/tools builds it from source checked against the hashes in
+# bin/tools/go.sum, so neither a developer machine nor the lint image needs
+# it installed.
 
 usage() {
     echo "usage: script/gofumpt --write|--check" >&2
@@ -22,15 +22,15 @@ usage() {
 
 main() {
     [ "$#" -eq 1 ] || usage
-    cd "$ROOT"
-    # Every Go file in the repo. gofumpt holds generated files, such as
-    # mfer/mf.pb.go, to gofmt's rules only.
+    cd "$ROOT/bin/tools"
+    # Every Go file in the repo, from $ROOT down. gofumpt holds generated
+    # files, such as mfer/mf.pb.go, to gofmt's rules only.
     case "$1" in
-        --write) go run "$GOFUMPT" -l -w . ;;
+        --write) go tool gofumpt -l -w "$ROOT" ;;
         --check)
             # Own line: a failing command inside `[ -n "$(...)" ]` does
             # not trip `set -e`, so a gofumpt that never ran would pass.
-            unformatted="$(go run "$GOFUMPT" -l .)"
+            unformatted="$(go tool gofumpt -l "$ROOT")"
             if [ -n "$unformatted" ]; then
                 echo "gofumpt: files need formatting (run make fmt):" >&2
                 echo "$unformatted" >&2