Install node, yarn and python from the distribution (closes #146)

Follows sneak's ruling at
#146 (comment). Runtimes
and the installer tools bundled with python come from the distribution,
with no version and no hash. Third-party packages stay pinned by hash.

script/bootstrap drops nvm and corepack, installs node and yarn from the
package manager, and stops when yarn's major version is not 1. This
repository's Dockerfile starts from Alpine 3.24. The Python example starts
from debian:trixie-slim and sets PIP_ONLY_BINARY=:all:, so pip downloads no
build tools without a hash check.

Model: opus-5-5

script/fmt

@@ -4,33 +4,13 @@ set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Must match the pin in script/bootstrap.
-NODE_VERSION="22.17.0"
-
-# True when the node on PATH has the pinned major version: the same
-# function as in script/bootstrap, which uses it to decide where it
-# installs yarn.
-node_is_pinned_major() {
-    if ! command -v node >/dev/null 2>&1; then return 1; fi
-    installed="$(node --version)"
-    installed="${installed#v}"
-    [ "${installed%%.*}" = "${NODE_VERSION%%.*}" ]
-}
-
-# Run the yarn script/bootstrap installed: the yarn on PATH when the
-# node on PATH has the pinned major version, and otherwise yarn under
-# the pinned node in nvm, which bootstrap leaves off the PATH of the
-# shell that called it. nvm is a bash script, hence the subshell.
+# Debian and Ubuntu name the yarn command yarnpkg.
 run_yarn() {
-    if node_is_pinned_major && command -v yarn >/dev/null 2>&1; then
-        exec yarn "$@"
+    if command -v yarn >/dev/null 2>&1; then
+        yarn "$@"
+    else
+        yarnpkg "$@"
     fi
-    if node_is_pinned_major || [ ! -s "$HOME/.nvm/nvm.sh" ]; then
-        echo "fmt: no yarn; run script/bootstrap first" >&2
-        exit 1
-    fi
-    exec bash -c '. "$HOME/.nvm/nvm.sh" && nvm use "$1" >/dev/null &&
-        shift && exec yarn "$@"' bash "$NODE_VERSION" "$@"
 }
 
 main() {