Add GitHub issue deduplication automation (#23926)

## Summary

This PR adds a Claude Code-powered issue deduplication system to help
reduce duplicate issues in the Bun repository.

### What's included:

1. **`/dedupe` slash command** (`.claude/commands/dedupe.md`)
- Claude Code command to find up to 3 duplicate issues for a given
GitHub issue
   - Uses parallel agent searches with diverse keywords
   - Filters out false positives

2. **Automatic dedupe on new issues**
(`.github/workflows/claude-dedupe-issues.yml`)
   - Runs automatically when a new issue is opened
   - Can also be triggered manually via workflow_dispatch
   - Uses the Claude Code base action to run the `/dedupe` command

3. **Auto-close workflow**
(`.github/workflows/auto-close-duplicates.yml`)
   - Runs daily to close issues marked as duplicates after 3 days
   - Only closes if:
     - Issue has a duplicate detection comment from bot
     - Comment is 3+ days old
     - No comments or activity after duplicate comment
     - Author hasn't reacted with 👎 to the duplicate comment

4. **Auto-close script** (`scripts/auto-close-duplicates.ts`)
   - TypeScript script that handles the auto-closing logic
   - Fetches open issues and checks for duplicate markers
   - Closes issues with proper labels and notifications

### How it works:

1. When a new issue is opened, the workflow runs Claude Code to analyze
it
2. Claude searches for duplicates and comments on the issue if any are
found
3. Users have 3 days to respond if they disagree
4. After 3 days with no activity, the issue is automatically closed

### Requirements:

- `ANTHROPIC_API_KEY` secret needs to be set in the repository settings
for the dedupe workflow to run

## Test plan

- [x] Verified workflow files have correct syntax
- [x] Verified script references correct repository (oven-sh/bun)
- [x] Verified slash command matches claude-code implementation
- [ ] Test workflow manually with workflow_dispatch (requires
ANTHROPIC_API_KEY)
- [ ] Monitor initial runs to ensure proper behavior

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>

.github/workflows/auto-close-duplicates.yml

@@ -0,0 +1,29 @@
+name: Auto-close duplicate issues
+on:
+  schedule:
+    - cron: "0 9 * * *"
+  workflow_dispatch:
+
+jobs:
+  auto-close-duplicates:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    concurrency:
+      group: auto-close-duplicates-${{ github.repository }}
+      cancel-in-progress: true
+    permissions:
+      contents: read
+      issues: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Setup Bun
+        uses: ./.github/actions/setup-bun
+
+      - name: Auto-close duplicate issues
+        run: bun run scripts/auto-close-duplicates.ts
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GITHUB_REPOSITORY: ${{ github.repository }}

.github/workflows/claude-dedupe-issues.yml

@@ -0,0 +1,34 @@
+name: Claude Issue Dedupe
+on:
+  issues:
+    types: [opened]
+  workflow_dispatch:
+    inputs:
+      issue_number:
+        description: 'Issue number to process for duplicate detection'
+        required: true
+        type: string
+
+jobs:
+  claude-dedupe-issues:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    concurrency:
+      group: claude-dedupe-issues-${{ github.event.issue.number || inputs.issue_number }}
+      cancel-in-progress: true
+    permissions:
+      contents: read
+      issues: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Run Claude Code slash command
+        uses: anthropics/claude-code-base-action@beta
+        with:
+          prompt: "/dedupe ${{ github.repository }}/issues/${{ github.event.issue.number || inputs.issue_number }}"
+          anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}
+          claude_args: "--model claude-sonnet-4-5-20250929"
+          claude_env: |
+            GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}