Say script/bootstrap pins node and yarn only when absent (refs #7)

The README Entrypoints line and the TODO.md Completed Steps entry said
script/bootstrap installs a pinned node and yarn. It installs them at the
pinned versions only when they are missing and uses a node or yarn already
installed whatever its version, as its header comment says.

Model: opus-5-5

README.md

@@ -52,8 +52,9 @@ development workflow, and the Makefile targets are thin shims that call them. We
 provide:
 
 - `script/bootstrap` — install all development dependencies (go, Go module
-  download, a pinned node and yarn, and the prettier pinned in `package.json`
-  and `yarn.lock`); the linter is not installed on the host
+  download, node and yarn at pinned versions when absent, and the prettier
+  pinned in `package.json` and `yarn.lock`); a node or yarn already installed is
+  used whatever its version; the linter is not installed on the host
 - `script/setup` — make a fresh clone ready for development: runs
   `script/bootstrap`, then `script/install-precommit`
 - `script/projectname` — print the project name (used for the Docker image tags)