feat: Implement comprehensive status check violation detection

Overview

Resolves the issue #4 where pull requests with failing status checks (such as required CI jobs) were not surfacing violations in Watchflow, even when corresponding rules were present and enabled in .watchflow/rules.yaml.

Changes

Core Infrastructure

• Added STATUS event type to EventType enum for handling GitHub status events
• Implemented StatusProcessor with agentic rule evaluation for GitHub status events
• Created StatusEventHandler for webhook routing with factory registration
• Enhanced event processing pipeline to support status check workflows

GitHub API Integration

• Enhanced GitHub API client with get_pr_checks() method supporting both:
  • Modern Check Runs API (GitHub Apps)
  • Legacy Status API (older integrations)
• Unified data format that normalizes both check types for consistent rule evaluation
• Upgraded PullRequestProcessor to fetch and include status check data in rule evaluation

Rule Validation System

• Implemented RequiredChecksValidator with comprehensive support for:
  • Check run API data (newer GitHub Apps)
  • Legacy status API data (commit statuses)
  • Mixed environments with both check types
  • Missing check scenarios and error handling
  • Different check conclusion/status combinations

Documentation & Testing

• Added comprehensive test suite with 6 test cases covering all validator scenarios
• Updated getting-started documentation with status check rule examples
• Enhanced development guide with pre-commit hooks setup instructions
• Created working examples for common CI/CD scenarios

Key Features

• Dual API Support: Works with both modern Check Runs API and legacy Status API
• Flexible Rule Configuration: Supports exact check names and pattern matching
• Comprehensive Error Handling: Graceful fallbacks when API calls fail
• Agentic Evaluation: Uses RuleEngineAgent for intelligent rule processing
• Backward Compatible: Doesn't break existing functionality

Usage Example

Users can now create rules like this in .watchflow/rules.yaml:

rules:
  - name: "Require CI Tests"
    description: "All CI tests must pass before merging"
    event_types: ["pull_request"]
    conditions:
      required_checks:
        - "ci/test"
        - "build"
        - "lint"

  - name: "Security Checks Required"
    description: "Security scans must pass"
    event_types: ["pull_request"]
    conditions:
      required_checks:
        - "security/scan"
        - "vulnerability-check"

  - name: "Code Quality Gates"
    description: "Code quality checks must pass"
    event_types: ["pull_request"]
    conditions:
      required_checks:
        - "codecov/patch"
        - "codeclimate"
        - "sonarqube"

Deployment Notes

This is a backward-compatible feature addition. Existing rules and functionality remain unchanged. New status check rules can be added to repositories without affecting existing workflows.