🐛 Refresh origin/HEAD before pubv preflight

pubv resolves the default branch from the local refs/remotes/origin/HEAD,
which git fetch never updates, so a clone or default-branch change left it
stale and pubv warned that main was not the default. Refresh it from the
remote before pubv runs, and assert releases are cut from main explicitly.

scripts/release.sh

@@ -23,12 +23,35 @@ set -eu
 # exactly what this ~30-line version replaces.
 
 CHANGELOG="CHANGELOG.md"
+BASE="main"
 
 if ! command -v code >/dev/null 2>&1; then
     echo "Error: 'code' (VS Code CLI) not found; install it or remove the editor step." >&2
     exit 1
 fi
 
+# Releases are cut from `main` (see CONTRIBUTING § Publishing workflow). Make
+# that explicit rather than relying on pubv's default-branch check, so the
+# error names `main` even when the remote's default is configured differently.
+CURRENT=$(git symbolic-ref --quiet --short HEAD || true)
+if [ "${CURRENT}" != "${BASE}" ]; then
+    echo "Error: releases are cut from '${BASE}', but HEAD is '${CURRENT:-detached}'." >&2
+    exit 1
+fi
+
+# pubv decides the "default branch" by reading the *local*
+# `refs/remotes/origin/HEAD`, not by asking the remote, and `git fetch` never
+# updates that ref. After a default-branch change — or a clone from when the
+# default was different — it goes stale and pubv warns/fails because the
+# current branch (main) does not match it, even though main *is* the remote
+# default. Refresh it from the remote first, so pubv's branch preflight
+# compares against reality. (Without a network this fails, but so would the
+# push pubv is about to do, so it is a real error rather than one to swallow.)
+if ! git remote set-head origin --auto >/dev/null 2>&1; then
+    echo "Error: could not refresh origin/HEAD; check connectivity to origin." >&2
+    exit 1
+fi
+
 echo "Running pubv..."
 pubv --no-tag --no-push --tag-prefix=none