Changes

PR #1783 by ksylvan: Update anthropic-sdk-go and add claude-sonnet-4-5

• Updated anthropic-sdk-go to version 1.13.0 for improved compatibility and performance
• Added support for ModelClaudeSonnet4_5 to the list of available AI models

Direct commits

• Added new extract_characters system definition with comprehensive character extraction capabilities
• Implemented canonical naming and deduplication rules for consistent character identification
• Created structured output schema with detailed formatting guidelines and examples
• Established interaction mapping functionality to track character relationships and narrative importance
• Added fallback handling for scenarios where no characters are found in the content