Closes claude-builders-bounty#4

## What this adds

-  — CLI tool that reviews GitHub PRs
-  — GitHub Action for automated reviews
-  — Full documentation with install & usage
-  — Sample outputs from 2 real PRs

## Features

- Accepts PR URL as input, fetches diff via GitHub API
- Static analysis: security patterns, size, test coverage, dependencies
- Structured Markdown output: summary, risks, suggestions, confidence score
- Zero dependencies (pure Python stdlib)
- GitHub Action workflow included

## Sample Reviews

1. **psf/requests#7310** — DigestAuth FIPS fix (High confidence, 1 file)
2. **fastapi/fastapi#15263** — pygments bump (High confidence, 1 file)