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/remove-autoclose-label.yml

@@ -1,6 +1,8 @@
-# Removes the "autoclose" label whenever a human (non-bot) posts a new comment
-# on an issue that carries the label.  This resets the inactivity clock.
-name: Remove Autoclose Label on Human Activity
+# Removes the "duplicate" label when a human (non-bot) comments on a
+# duplicate-flagged issue, signaling that the issue needs re-evaluation.
+# The auto-close script also independently checks for human activity,
+# so this provides an additional visible signal.
+name: Remove Duplicate Label on Human Activity
 
 on:
   issue_comment:
@@ -10,39 +12,34 @@ permissions:
   issues: write
 
 jobs:
-  remove-autoclose:
+  remove-label:
     # Only run for issue comments (not PR comments)
-    if: ${{ github.event.issue.pull_request == null }}
+    if: >
+      github.event.issue.pull_request == null &&
+      !endsWith(github.actor, '[bot]') &&
+      github.actor != 'github-actions'
     runs-on: ubuntu-latest
     steps:
-      - name: Remove autoclose label if human commented
+      - name: Remove duplicate label if human commented
         uses: actions/github-script@v7
         with:
           script: |
-            const actor = context.actor;
-
-            // Ignore bot accounts
-            if (actor.endsWith('[bot]') || actor === 'github-actions') {
-              core.info(`Skipping bot comment from ${actor}`);
-              return;
-            }
-
-            const issue  = context.payload.issue;
+            const issue = context.payload.issue;
             const labels = (issue.labels || []).map(l => l.name);
 
-            if (!labels.includes('autoclose')) {
-              core.info('Issue does not have "autoclose" label – nothing to do.');
+            if (!labels.includes('duplicate')) {
+              core.info('Issue does not have "duplicate" label - nothing to do.');
               return;
             }
 
             await github.rest.issues.removeLabel({
-              owner:        context.repo.owner,
-              repo:         context.repo.repo,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
               issue_number: issue.number,
-              name:         'autoclose',
+              name: 'duplicate',
             });
 
             core.info(
-              `Removed "autoclose" label from #${issue.number} ` +
-              `after human activity by ${actor}`
+              `Removed "duplicate" label from #${issue.number} ` +
+              `after human comment by ${context.actor}`
             );