feat: add bin/gstack-pr-title-rewrite.sh shared helper

Single source of truth for "rewrite a PR title to start with v<VERSION>".
Three cases: already correct (no-op), different prefix (replace), no prefix
(prepend). Rejects malformed VERSION (anything outside ^[0-9]+(\.[0-9]+)*$)
with exit code 2. Uses literal case prefix match instead of bash's pattern-
matching # operator so a VERSION with glob metacharacters cannot mismatch.

Free bun test covers the four branches plus malformed-input rejection,
plain-words-not-stripped, single-segment-not-stripped, idempotence, and
missing-args. 9 tests, ~400ms.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

bin/gstack-pr-title-rewrite.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+# Rewrite a PR/MR title to start with v<NEW_VERSION>.
+#
+# Usage:  bin/gstack-pr-title-rewrite.sh <NEW_VERSION> <CURRENT_TITLE>
+# Output: corrected title on stdout.
+#
+# Rule: PR titles MUST start with v<NEW_VERSION>. Three cases:
+#   1. Already starts with "v<NEW_VERSION> " -> no change.
+#   2. Starts with a different "v<digits and dots> " prefix -> replace prefix.
+#   3. No version prefix -> prepend "v<NEW_VERSION> ".
+#
+# The version-prefix regex matches two or more dot-separated digit segments
+# (covers v1.2, v1.2.3, v1.2.3.4) so the rule is portable across repos that
+# use 3-part or 4-part versions, but does NOT strip plain words like
+# "version 5".
+
+set -euo pipefail
+
+if [ $# -lt 2 ]; then
+  echo "usage: $0 <NEW_VERSION> <CURRENT_TITLE>" >&2
+  exit 2
+fi
+
+NEW_VERSION="$1"
+TITLE="$2"
+
+# Reject malformed NEW_VERSION early. Real values are dot-separated digits;
+# anything with shell pattern metacharacters or whitespace is a caller bug.
+if ! printf '%s' "$NEW_VERSION" | grep -qE '^[0-9]+(\.[0-9]+)*$'; then
+  echo "error: NEW_VERSION must be dot-separated digits, got: $NEW_VERSION" >&2
+  exit 2
+fi
+
+# Literal prefix match (case statement is glob-quoted by bash, but our
+# regex-validated NEW_VERSION has no glob metacharacters so this is safe).
+case "$TITLE" in
+  "v$NEW_VERSION "*)
+    printf '%s\n' "$TITLE"
+    exit 0
+    ;;
+esac
+
+REST=$(printf '%s' "$TITLE" | sed -E 's/^v[0-9]+(\.[0-9]+)+ //')
+printf 'v%s %s\n' "$NEW_VERSION" "$REST"