feat: add GitHub App PR verification example

[nexicturbo]

Closes #28

Summary

• Add a self-contained GitHub App example under examples/platform-integrations/github-app.
• Verify pull_request webhooks against configurable APort policy packs.
• Create GitHub Check Runs with success/failure/neutral conclusions on the PR head commit.
• Include environment-based policy configuration, webhook signature handling, installation auth, error handling, and setup docs.
• List the GitHub PR verifier in the root integration table.

Verification

• npm test
• GITHUB_APP_ID=123 GITHUB_WEBHOOK_SECRET=secret GITHUB_PRIVATE_KEY='-----BEGIN RSA PRIVATE KEY-----\nabc\n-----END RSA PRIVATE KEY-----' APORT_API_KEY=test APORT_AGENT_ID=agt node -e "require('./src/server'); console.log('server module loaded')"
• node --check src/aport-client.js && node --check src/config.js && node --check src/pull-request-context.js && node --check src/check-run.js && node --check src/verifier.js && node --check src/server.js && node --check tests/aport-client.test.js && node --check tests/verifier.test.js
• npm pack --dry-run
• git diff --check

Bounty

This PR is for the $20 USD bounty in #28.