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>

internal/config/models_test.go

@@ -10,35 +10,71 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func TestModelsResolve(t *testing.T) {
-	yaml := `
-default: ollama/default-model
+const testYAML = `
+verifier: claude-sonnet-4-6
+llama_swap_url: http://koala:8080
+
+default_chain:
+  - ollama/qwen3-coder-30b-tuned
+  - claude-sonnet-4-6
+
 skills:
-  tdd: ollama/qwen3-coder-30b-tuned
-  review: ollama/devstral-tuned
+  review:
+    chain:
+      - ollama/devstral-tuned
+      - ollama/gemma4
+      - claude-sonnet-4-6
+  spec:
+    chain:
+      - ollama/phi4
+      - claude-opus-4-6
 `
+
+func writeModels(t *testing.T, content string) string {
+	t.Helper()
 	f := filepath.Join(t.TempDir(), "models.yaml")
-	require.NoError(t, os.WriteFile(f, []byte(yaml), 0644))
-
-	m, err := config.LoadModels(f)
-	require.NoError(t, err)
-
-	assert.Equal(t, "ollama/qwen3-coder-30b-tuned", m.Resolve("tdd", ""))
-	assert.Equal(t, "ollama/devstral-tuned", m.Resolve("review", ""))
-	assert.Equal(t, "ollama/default-model", m.Resolve("unknown", ""))
+	require.NoError(t, os.WriteFile(f, []byte(content), 0644))
+	return f
 }
 
-func TestModelsOverride(t *testing.T) {
-	yaml := `
-default: ollama/default-model
-skills:
-  tdd: ollama/qwen3-coder-30b-tuned
-`
-	f := filepath.Join(t.TempDir(), "models.yaml")
-	require.NoError(t, os.WriteFile(f, []byte(yaml), 0644))
+func TestModelsVerifier(t *testing.T) {
+	m, err := config.LoadModels(writeModels(t, testYAML))
+	require.NoError(t, err)
+	assert.Equal(t, "claude-sonnet-4-6", m.Verifier())
+}
 
-	m, err := config.LoadModels(f)
+func TestModelsLlamaSwapURL(t *testing.T) {
+	m, err := config.LoadModels(writeModels(t, testYAML))
+	require.NoError(t, err)
+	assert.Equal(t, "http://koala:8080", m.LlamaSwapURL())
+}
+
+func TestModelsChainForSkillOverride(t *testing.T) {
+	m, err := config.LoadModels(writeModels(t, testYAML))
 	require.NoError(t, err)
 
-	assert.Equal(t, "anthropic/claude-sonnet-4-6", m.Resolve("tdd", "anthropic/claude-sonnet-4-6"))
+	chain := m.ChainFor("review", "")
+	require.Len(t, chain, 3)
+	assert.Equal(t, "ollama/devstral-tuned", chain[0])
+	assert.Equal(t, "ollama/gemma4", chain[1])
+	assert.Equal(t, "claude-sonnet-4-6", chain[2])
+}
+
+func TestModelsChainForDefaultFallback(t *testing.T) {
+	m, err := config.LoadModels(writeModels(t, testYAML))
+	require.NoError(t, err)
+
+	chain := m.ChainFor("trainer", "") // not in skills map
+	require.Len(t, chain, 2)
+	assert.Equal(t, "ollama/qwen3-coder-30b-tuned", chain[0])
+	assert.Equal(t, "claude-sonnet-4-6", chain[1])
+}
+
+func TestModelsChainForCallerOverride(t *testing.T) {
+	m, err := config.LoadModels(writeModels(t, testYAML))
+	require.NoError(t, err)
+
+	chain := m.ChainFor("review", "claude-opus-4-6")
+	require.Len(t, chain, 1)
+	assert.Equal(t, "claude-opus-4-6", chain[0])
 }