Add Greptile codebase indexing for AI-powered PR review

[Jesssullivan]

Set up Greptile for codebase-aware AI PR review.

Steps

1. Go to https://app.greptile.com
2. Connect GitHub account and select jesssullivan/zig-ctap2
3. Install the Greptile GitHub App
4. Configure auto-commenting on PRs

CodeRabbit is already configured via .coderabbit.yaml.