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/autoclose-labeled-issues.yml

@@ -1,21 +1,14 @@
-# Closes open issues that carry the "autoclose" label and have been inactive
-# for more than INACTIVITY_DAYS days.  Runs on a daily schedule and can also
-# be triggered manually.
-name: Auto-close Inactive Labeled Issues
+# Auto-closes duplicate issues after 3 days if no human activity or thumbs-down reaction.
+# Runs daily at 09:00 UTC.
+name: Auto-close Duplicate Issues
 
 on:
   schedule:
-    # Runs every day at 01:00 UTC
-    - cron: '0 1 * * *'
+    - cron: '0 9 * * *'
   workflow_dispatch:
     inputs:
-      inactivity_days:
-        description: 'Days of inactivity before closing (default: 7)'
-        required: false
-        default: '7'
-        type: number
       dry_run:
-        description: 'Dry run – report but do not actually close issues'
+        description: 'Dry run - report but do not close issues'
         required: false
         default: 'false'
         type: choice
@@ -30,16 +23,15 @@ permissions:
 jobs:
   autoclose:
     runs-on: ubuntu-latest
+    timeout-minutes: 10
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
 
-      - name: Close inactive autoclose-labeled issues
+      - name: Close inactive duplicate issues
         env:
-          GITHUB_TOKEN:     ${{ secrets.GITHUB_TOKEN }}
-          REPO_OWNER:       ${{ github.repository_owner }}
-          REPO_NAME:        ${{ github.event.repository.name }}
-          # workflow_dispatch overrides the default; schedule uses the default (7)
-          INACTIVITY_DAYS:  ${{ inputs.inactivity_days || '7' }}
-          DRY_RUN:          ${{ inputs.dry_run || 'false' }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          REPO_OWNER: ${{ github.repository_owner }}
+          REPO_NAME: ${{ github.event.repository.name }}
+          DRY_RUN: ${{ inputs.dry_run || 'false' }}
         run: node scripts/autoclose-labeled-issues.js