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>
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