Refactor issue dedup system to use claude-code-action with /dedupe command

Replace the copilot-generated inline search logic with a claude-code-action
based architecture inspired by anthropic/claude-code's approach:

- Add .claude/commands/dedupe.md with 5-parallel-search strategy
- Add scripts/comment-on-duplicates.sh with 3-day grace period warning
- Rewrite issue-dedupe.yml to use claude-code-action + /dedupe command
- Rewrite autoclose script to check bot comments, human activity, and thumbsdown
- Rewrite backfill to trigger dedupe workflow per issue with rate limiting
- Add concurrency control, timeout, input validation, and rate limit retry
- Remove gh.sh (unnecessary), backfill-dedupe.js (replaced by workflow trigger)

.github/workflows/backfill-dedupe.yml

@@ -1,4 +1,4 @@
-# Backfills duplicate detection for historical issues.
+# Backfills duplicate detection for historical issues using Claude Code.
 # Triggered manually via workflow_dispatch.
 name: Backfill Duplicate Detection
 
@@ -10,58 +10,55 @@ on:
         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
+  issues: write
+  actions: write
 
 jobs:
   backfill:
     runs-on: ubuntu-latest
+    timeout-minutes: 10
     steps:
-      - name: Checkout repository
-        uses: actions/checkout@v4
+      - 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
+      - name: Fetch issues and run dedupe
         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
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          REPO: ${{ github.repository }}
+          DAYS_BACK: ${{ inputs.days_back || '30' }}
+        run: |
+          if ! [[ "$DAYS_BACK" =~ ^[0-9]+$ ]]; then
+            echo "Error: days_back must be a number"
+            exit 1
+          fi
+
+          SINCE=$(date -u -d "$DAYS_BACK days ago" +%Y-%m-%dT%H:%M:%SZ 2>/dev/null || date -u -v-${DAYS_BACK}d +%Y-%m-%dT%H:%M:%SZ)
+          echo "Fetching open issues since $SINCE"
+
+          # Get open issues, filter out PRs and already-labeled ones
+          ISSUES=$(gh issue list --repo "$REPO" --state open --limit 200 --json number,title,labels,createdAt \
+            --jq "[.[] | select(.createdAt >= \"$SINCE\") | select([.labels[].name] | index(\"duplicate\") | not)] | .[].number")
+
+          if [ -z "$ISSUES" ]; then
+            echo "No issues to process"
+            exit 0
+          fi
+
+          BATCH_SIZE=10
+          COUNT=0
+          echo "Issues to process: $ISSUES"
+          for NUMBER in $ISSUES; do
+            echo "Triggering dedupe for issue #$NUMBER"
+            gh workflow run issue-dedupe.yml --repo "$REPO" -f issue_number="$NUMBER"
+            COUNT=$((COUNT + 1))
+            if [ $((COUNT % BATCH_SIZE)) -eq 0 ]; then
+              echo "Pausing 60s after $COUNT issues..."
+              sleep 60
+            else
+              sleep 5
+            fi
+          done
+
+          echo "Backfill triggered for $COUNT issues"