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.go

@@ -7,9 +7,15 @@ import (
 	"gopkg.in/yaml.v3"
 )
 
+type skillChain struct {
+	Chain []string `yaml:"chain"`
+}
+
 type modelsFile struct {
-	Default string            `yaml:"default"`
-	Skills  map[string]string `yaml:"skills"`
+	Verifier     string                `yaml:"verifier"`
+	LlamaSwapURL string                `yaml:"llama_swap_url"`
+	DefaultChain []string              `yaml:"default_chain"`
+	Skills       map[string]skillChain `yaml:"skills"`
 }
 
 type Models struct {
@@ -28,16 +34,23 @@ func LoadModels(path string) (Models, error) {
 	return Models{data: f}, nil
 }
 
-// Resolve returns the model for a skill, respecting three-layer priority:
-// 1. override (from MCP call) — highest
-// 2. per-skill default from models.yaml
-// 3. global default
-func (m Models) Resolve(skill, override string) string {
+// Verifier returns the model name to use for all local-tier output verification.
+func (m Models) Verifier() string { return m.data.Verifier }
+
+// LlamaSwapURL returns the llama-swap base URL for warm-state probing.
+func (m Models) LlamaSwapURL() string { return m.data.LlamaSwapURL }
+
+// ChainFor returns the ordered list of model names for a skill.
+// If override is non-empty, returns a single-entry chain (no escalation).
+// Falls back to default_chain when the skill has no explicit entry.
+func (m Models) ChainFor(skill, override string) []string {
 	if override != "" {
-		return override
+		return []string{override}
 	}
-	if model, ok := m.data.Skills[skill]; ok {
-		return model
+	if sc, ok := m.data.Skills[skill]; ok && len(sc.Chain) > 0 {
+		return sc.Chain
 	}
-	return m.data.Default
+	out := make([]string, len(m.data.DefaultChain))
+	copy(out, m.data.DefaultChain)
+	return out
 }