Pin the remaining developer tool installs (closes #68)

gofumpt was pinned only by its version tag. It is now a tool of a
separate module in tools/, so `go tool` builds it from source checked
against the hashes in tools/go.sum, and mfer's own module gains no
dependencies. script/prettier no longer falls back to a prettier on
PATH, and fails when node_modules holds a different version than
package.json pins. The bootstrap comment now says what --frozen-lockfile
really does, package.json drops its made-up version, and the golang
image comment names its exact version.

Model: opus-5-5

script/prettier

@@ -18,24 +18,9 @@ usage() {
     exit 2
 }
 
-# Prefer the version pinned by package.json/yarn.lock so that CI and
-# developer machines format identically. Fall back to a prettier on PATH,
-# but say so, because a different version formats differently.
-find_prettier() {
-    if [ -x "$ROOT/node_modules/.bin/prettier" ]; then
-        printf '%s\n' "$ROOT/node_modules/.bin/prettier"
-        return 0
-    fi
-    if command -v prettier >/dev/null 2>&1; then
-        echo "prettier: node_modules/.bin/prettier is absent; using the" \
-            "prettier on PATH, which may be a different version than the" \
-            "one pinned in package.json. Run script/bootstrap to install" \
-            "the pinned version." >&2
-        command -v prettier
-        return 0
-    fi
-    return 1
-}
+# Only the prettier yarn installed from yarn.lock, never one on PATH: a
+# different version formats differently.
+PRETTIER="$ROOT/node_modules/.bin/prettier"
 
 main() {
     [ "$#" -eq 1 ] || usage
@@ -46,10 +31,17 @@ main() {
 
     cd "$ROOT"
 
-    if ! prettier_bin="$(find_prettier)"; then
-        echo "prettier: not found." >&2
-        echo "  Install it with: script/bootstrap" >&2
-        echo "  (installs the version pinned in package.json/yarn.lock)" >&2
+    # node_modules keeps the old prettier after package.json moves to a new
+    # one, until script/bootstrap runs again, so compare the two. A prettier
+    # that runs also means node is on PATH, which reading package.json needs.
+    if ! installed="$("$PRETTIER" --version 2>/dev/null)"; then
+        echo "prettier: not installed; run script/bootstrap" >&2
+        exit 1
+    fi
+    pinned="$(node -p 'require("./package.json").devDependencies.prettier')"
+    if [ "$installed" != "$pinned" ]; then
+        echo "prettier: package.json pins $pinned but $installed is" \
+            "installed; run script/bootstrap" >&2
         exit 1
     fi
 
@@ -62,8 +54,8 @@ main() {
     # patterns always match at least one tracked file (README.md,
     # package.json), so an empty match means the glob broke, and prettier
     # erroring out is exactly what we want rather than a vacuous pass.
-    "$prettier_bin" "$mode" "**/*.md"
-    "$prettier_bin" "$mode" "**/*.json"
+    "$PRETTIER" "$mode" "**/*.md"
+    "$PRETTIER" "$mode" "**/*.json"
 }
 
 main "$@"