Implement core AI service infrastructure including agent operations, escalation handling, and asynchronous queue processing.
New services:
- Agent service: Handles Tier 2 AI operations with action runner integration
- Queue service: Asynchronous processing of AI operations with retry logic
- Escalation service: Routes complex issues to staff with configurable rules
- Notifier service: Sends notifications for AI operation results
Additional changes:
- Add GitCaddy AI system user (ID: -3) for bot operations
- Add AIConfig to repository units
- Add AI-specific error codes (rate limiting, service errors, etc.)
- Extend AI client with GenerateIssueResponse method
- Add AISettingsV2 struct for repository-level AI configuration
The queue system enables non-blocking AI operations with proper error handling and rate limiting.