Break up the oversized runtime.go (2510 lines) by extracting cohesive
groups of functions into dedicated files, and move pure error
classification utilities into a new reusable package.
New files in pkg/runtime:
- model_picker.go: model change/revert tool handlers
- agent_delegation.go: sub-agent dispatch, task transfer, handoffs
New package pkg/modelerrors:
- ContextOverflowError type and detection
- HTTP status code extraction (Anthropic, Gemini, OpenAI)
- Retryable error classification
- Exponential backoff with jitter
- User-friendly error formatting
Assisted-By: docker-agent
