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/bootstrap

@@ -132,15 +132,18 @@ main() {
     # ---- JS / docs repos ----
     # This is a Go repo, but node and yarn are required anyway: prettier
     # formats the Markdown and JSON, and script/fmt-check verifies it.
-    # The version is pinned by package.json/yarn.lock, whose integrity
-    # hashes --frozen-lockfile enforces.
+    # The version is pinned by package.json/yarn.lock: yarn checks every
+    # package it fetches against its yarn.lock integrity hash, and
+    # --frozen-lockfile fails instead of rewriting a yarn.lock that no
+    # longer matches package.json.
     ensure_node
     ensure_yarn
     install_js_deps
 
     # ---- Go repos ----
     if missing go; then pkg_install go golang go go; fi
-    # No golangci-lint: script/lint runs it in Docker only.
+    # No golangci-lint: script/lint runs it in Docker only. No gofumpt:
+    # script/gofumpt builds the version tools/go.mod requires.
     go mod download
 
     # ---- Python repos ----