feat: add AI-powered duplicate PR detection via opencode

Same approach as the duplicate issue detector — uses opencode run
with gh pr commands to find potentially duplicate open PRs when
external contributors open new PRs. Skips core team and bots.

.github/workflows/check_duplicate_prs.yml

@@ -0,0 +1,58 @@
+name: Check Duplicate PRs
+
+on:
+  pull_request_target:
+    types: [opened]
+
+jobs:
+  check-duplicates:
+    if: |
+      github.event.pull_request.user.login != 'ishaan-jaff' &&
+      github.event.pull_request.user.login != 'krrishdholakia' &&
+      github.event.pull_request.user.login != 'actions-user' &&
+      !endsWith(github.event.pull_request.user.login, '[bot]')
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
+    steps:
+      - name: Install opencode
+        run: curl -fsSL https://opencode.ai/install | bash
+
+      - name: Check duplicates
+        env:
+          ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          OPENCODE_PERMISSION: |
+            {
+              "bash": {
+                "*": "deny",
+                "gh pr*": "allow"
+              },
+              "webfetch": "deny"
+            }
+        run: |
+          opencode run -m anthropic/claude-sonnet-4-6 "A new PR has been opened:
+
+          PR number: ${{ github.event.pull_request.number }}
+
+          Lookup this PR with gh pr view ${{ github.event.pull_request.number }}.
+
+          Search through existing open PRs (excluding #${{ github.event.pull_request.number }}) to find potential duplicates.
+
+          Use gh pr list with relevant search terms from the new PR's title and description. Try multiple keyword combinations to search broadly. Check both open and recently closed PRs.
+
+          Consider:
+          1. Similar titles or descriptions
+          2. Same bug fix or feature being implemented
+          3. Related functionality or components
+          4. Overlapping code changes (same files or areas)
+
+          If you find potential duplicates, post a SINGLE comment on PR #${{ github.event.pull_request.number }} using gh pr comment with this format:
+
+          _This comment was generated by an LLM and may be inaccurate._
+
+          This PR might be a duplicate of existing PRs. Please check:
+          - #[pr_number]: [brief description of similarity]
+
+          If you find NO duplicates, do NOT post any comment. Stay silent."