ci(issues): flag likely-duplicate issues on open

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.

.github/workflows/detect-duplicate.yml

@@ -0,0 +1,38 @@
+name: 🔁 Detect Duplicate Issues
+
+on:
+  issues:
+    types: [opened]
+
+permissions:
+  contents: read
+
+concurrency:
+  group: detect-duplicate-${{ github.event.issue.number }}
+  cancel-in-progress: true
+
+jobs:
+  detect:
+    name: 🔍 Find similar issues
+    if: github.actor != 'github-actions[bot]'
+    runs-on: ubuntu-24.04
+    permissions:
+      issues: write
+      contents: read
+    steps:
+      - name: 📥 Checkout repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: 🍞 Setup Bun
+        uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2.2.0
+        with:
+          bun-version: latest
+
+      - name: 🔍 Detect duplicate issues
+        run: bun scripts/detect-duplicate-issue.mjs
+        env:
+          GH_TOKEN: ${{ github.token }}
+          GITHUB_REPOSITORY: ${{ github.repository }}
+          ISSUE_NUMBER: ${{ github.event.issue.number }}
+          ISSUE_TITLE: ${{ github.event.issue.title }}
+          ISSUE_BODY: ${{ github.event.issue.body }}