Implements escalation chains per skill with three-layer priority:
1. Caller override (model param) — no escalation
2. Per-skill chain from models.yaml
3. default_chain fallback
New APIs:
- Verifier() — fixed verifier for output validation
- LlamaSwapURL() — base URL for warm-state probing
- ChainFor(skill, override) — ordered model list for escalation
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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