Adds .github/workflows/detect-duplicate.yml + scripts/detect-duplicate-issue.mjs
(Bun, dep-free, no API key): on a new issue, compares its title/body to open
issues via Jaccard similarity (with light stemming and stop-words), and if the
top matches pass a threshold, posts one comment listing them and adds a
'possible duplicate' label. Inspired by seerr's detect-duplicate, minus the
embedding/Groq dependency.
