Add provider smokes and model preference

[bussyjd]

Summary

• Add obol model prefer <model-name> to move a configured LiteLLM model to the front of the preference order and sync stack-managed agent model config.
• Honor LiteLLM model order when choosing OpenClaw primary/fallback models, while keeping cloud setup primary by preferring the configured cloud model.
• Add flow-12 provider smokes for local obol-agent inference plus optional Anthropic/OpenAI smokes when API keys are present.

Tests

• bash -n flows/flow-12-agent-provider-smokes.sh flows/release-smoke.sh
• go test ./cmd/obol ./internal/model ./internal/openclaw
• go test ./... -run ^'$' -count=0

Closes #378