Variable classifyClaudeAxonEventConst

Classifies a raw Axon event into a ClaudeTimelineEvent.

Classification rules:

1. SYSTEM_EVENT with a recognized type (turn.started, turn.resumed, turn.completed, turn.failed, broker.error, background.changed, ...) -> system
2. Known Claude protocol event_type -> claude_protocol with eventType discriminator
3. Everything else -> unknown