Add GitHub Actions workflows for issue deduplication and auto-close

Co-authored-by: BukeLy <19304666+BukeLy@users.noreply.github.com>

.github/workflows/backfill-dedupe.yml

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+# Backfills duplicate detection for historical issues.
+# Triggered manually via workflow_dispatch.
+name: Backfill Duplicate Detection
+
+on:
+  workflow_dispatch:
+    inputs:
+      days_back:
+        description: 'How many days back to look for issues (default: 30)'
+        required: false
+        default: '30'
+        type: number
+      dry_run:
+        description: 'Dry run – analyze but do not post comments or apply labels'
+        required: false
+        default: 'false'
+        type: choice
+        options:
+          - 'false'
+          - 'true'
+
+permissions:
+  issues: write
+  contents: read
+
+jobs:
+  backfill:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Ensure required labels exist
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const labels = [
+              { name: 'duplicate', color: 'cfd3d7', description: 'This issue or pull request already exists' },
+              { name: 'autoclose', color: 'e4e669', description: 'Will be auto-closed after a period of inactivity' },
+            ];
+            for (const label of labels) {
+              try {
+                await github.rest.issues.getLabel({
+                  owner: context.repo.owner,
+                  repo:  context.repo.repo,
+                  name:  label.name,
+                });
+              } catch (err) {
+                if (err.status === 404) {
+                  await github.rest.issues.createLabel({
+                    owner: context.repo.owner, repo: context.repo.repo,
+                    name: label.name, color: label.color, description: label.description,
+                  });
+                  core.info(`Created label: ${label.name}`);
+                }
+              }
+            }
+
+      - name: Run backfill script
+        env:
+          GITHUB_TOKEN:      ${{ secrets.GITHUB_TOKEN }}
+          ANTHROPIC_API_KEY: ${{ secrets.AUTHROPIC_API_KEY }}
+          REPO_OWNER:        ${{ github.repository_owner }}
+          REPO_NAME:         ${{ github.event.repository.name }}
+          DAYS_BACK:         ${{ inputs.days_back }}
+          DRY_RUN:           ${{ inputs.dry_run }}
+        run: node scripts/backfill-dedupe.js