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

TODO.md

@@ -22,8 +22,9 @@ in the same commit, and fix the findings it surfaces
 
 - 2026-10-06: Formatted Markdown with prettier: `script/fmt` writes and
   `script/fmt-check` checks every Markdown file; prettier pinned in
-  `package.json` and `yarn.lock`, installed by `script/bootstrap` with a pinned
-  node and yarn; existing Markdown reformatted.
+  `package.json` and `yarn.lock`, installed by `script/bootstrap`, which
+  installs node and yarn at pinned versions when absent and otherwise uses the
+  ones already installed; existing Markdown reformatted.
 - 2026-10-05: Brought the repo up to the standard layout: canonical
   `.gitignore`, `.dockerignore` and `.editorconfig`; `lint` and `test` phases in
   the `Dockerfile`, built by `script/lint` and `script/test`; canonical