feat: Add PR Review Agent — structured Markdown PR review tool (#4)

[zlplzp123wyt]

🤖 PR Review Agent — Bounty Submission (claude-builders-bounty#4)

Closes claude-builders-bounty#4

What This Adds

• CLI tool (claude_review.py) — Reviews any GitHub PR and outputs structured Markdown
• GitHub Action (.github/workflows/pr-review.yml) — Auto-posts review comments on PR events
• Full README — Installation, usage, and configuration docs
• 2 real-world sample outputs demonstrating the tool

Features

• ✅ CLI: claude-review --pr https://github.com/owner/repo/pull/123
• ✅ GitHub Action workflow included
• ✅ Structured Markdown output with all required sections:
  • Summary of changes (2–3 sentences)
  • Identified risks (security, size, tests, deps, config)
  • Improvement suggestions
  • Confidence score (Low / Medium / High)
• ✅ Zero dependencies — pure Python standard library
• ✅ JSON output mode for programmatic use

Tested on Real PRs

1. psf/requests#7310 — DigestAuth FIPS fix

## 🤖 Automated PR Review

### 📋 Summary of Changes
"Move DigestAuth hash algorithms to use usedforsecurity=False" by @nateprewitt
modifies 1 file(s) (+5/−5 lines). Changes span: source code.

### ⚠️ Identified Risks
- 🧪 No tests: Source code changed but no test files updated

### 💡 Improvement Suggestions
- 👍 Code changes look clean

### 📊 Confidence Score
🟢 High

2. fastapi/fastapi#15263 — pygments bump

## 🤖 Automated PR Review

### 📋 Summary of Changes
"⬆ Bump pygments from 2.19.2 to 2.20.0" by @dependabot[bot]
modifies 1 file(s) (+3/−3 lines).

### ⚠️ Identified Risks
- 🧪 No tests: Source code changed but no test files updated

### 📊 Confidence Score
🟢 High

Project Structure

.github/workflows/pr-review.yml   # GitHub Action workflow
agents/pr-reviewer/
  ├── claude_review.py             # Main CLI tool
  ├── setup.py                     # pip install support
  ├── requirements.txt             # None! (stdlib only)
  ├── README.md                    # Documentation
  └── examples/
      ├── sample_review_1.md       # psf/requests#7310 output
      └── sample_review_2.md       # fastapi/fastapi#15263 output