Add GitHub Actions CI/CD Pipeline

Adds a comprehensive CI/CD workflow (.github/workflows/ci.yml) with three jobs:

### Test Job

- Runs deno lint to check code style
- Runs deno fmt --check to verify formatting
- Runs deno test with all required permissions

### Claude Code Review Job

- Automatically reviews PRs using Claude AI
- Checks for:
    - TypeScript strict mode compliance
    - Code style adherence (CLAUDE.md guidelines)
    - Domain-driven design principles
    - Test coverage requirements
    - Security vulnerabilities
    - Potential bugs and edge cases
- Posts review comments directly on PRs

### Auto-merge Job

- Automatically merges PRs when ALL checks pass (lint, test, format, Claude review)
- Only activates for:
    - Dependabot PRs (automatic)
    - PRs with the automerge label (manual)
- Uses squash merge strategy
- Security: Only works for same-repository PRs (not forks)