- Add ContextLength(ctx) int to Backend interface; each backend fetches
from its API (OpenAI /v1/models, Ollama /api/show, CodeWhisperer
ListAvailableModels) with caching. Falls back to 128k default.
- Add Models(ctx) []string to Backend interface for model discovery.
- Add /models command to list available models.
- Auto-compact before each turn when estimated tokens >= 75% of context length.