feat(config): replace single-model config with chain-based routing

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>

config/models.yaml

@@ -1,13 +1,41 @@
-# Model routing table — three-layer priority:
-# 1. model param in MCP tool call (caller override)
-# 2. per-skill entry here
-# 3. default (fallback)
-default: ollama/qwen3-coder-30b-tuned
+# Model routing chains — three-layer priority:
+# 1. model param in MCP tool call (caller override — collapses to single entry, no escalation)
+# 2. per-skill chain here
+# 3. default_chain fallback
+
+verifier: claude-sonnet-4-6   # fixed verifier for all local tiers
+
+llama_swap_url: http://koala:8080   # for warm-state probing
+
+default_chain:
+  - ollama/qwen3-coder-30b-tuned
+  - claude-sonnet-4-6
 
 skills:
-  tdd:           ollama/qwen3-coder-30b-tuned
-  review:        ollama/devstral-tuned
-  debug:         ollama/deepseek-r1-tuned
-  retrospective: ollama/qwen3-coder-30b-tuned
-  spec:          ollama/qwen3-coder-30b-tuned
-  trainer:       ollama/qwen3-coder-30b-tuned
+  tdd:
+    chain:
+      - ollama/qwen3-coder-30b-tuned
+      - claude-sonnet-4-6
+  review:
+    chain:
+      - ollama/devstral-tuned
+      - ollama/gemma4
+      - claude-sonnet-4-6
+  debug:
+    chain:
+      - ollama/deepseek-r1-tuned
+      - claude-sonnet-4-6
+  spec:
+    chain:
+      - ollama/phi4
+      - ollama/gemma4
+      - claude-sonnet-4-6
+      - claude-opus-4-6
+  retrospective:
+    chain:
+      - ollama/qwen3-coder-30b-tuned
+      - claude-sonnet-4-6
+  trainer:
+    chain:
+      - ollama/qwen3-coder-30b-tuned
+      - claude-sonnet-4-6