Add PR-Agent workflow and demo change

This PR demonstrates PR-Agent's automated review capabilities.
The workflow will automatically review PRs when they are opened.

.github/workflows/pr-agent.yml

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+name: PR Agent
+on:
+  pull_request:
+    types: [opened, synchronize, reopened, ready_for_review]
+jobs:
+  pr_agent_job:
+    runs-on: ubuntu-latest
+    steps:
+      - name: PR Agent action step
+        uses: Codium-ai/pr-agent@main
+        env:
+          OPENAI_KEY: ${{ secrets.OPENAI_KEY }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+

DEMO.md

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+# PR-Agent Demo
+
+This file was created to demonstrate PR-Agent's automated review capabilities.
+
+**Demo Change:** $(date -u)
+
+## What to Expect
+
+When this PR is opened, PR-Agent will:
+1. Automatically analyze the changes
+2. Generate a comprehensive review with:
+   - Estimated effort to review
+   - Security concerns
+   - Test coverage recommendations
+   - Code improvement suggestions
+
+You can also interact with PR-Agent by commenting:
+- `/review` - Get a review of the PR
+- `/improve` - Get code improvement suggestions
+- `/describe` - Auto-generate PR description
+- `/ask <question>` - Ask questions about the PR
+