feat(amp): add model mapping support for routing unavailable models to alternatives

- Add AmpModelMapping config to route models like 'claude-opus-4.5' to 'claude-sonnet-4'
- Add ModelMapper interface and DefaultModelMapper implementation with hot-reload support
- Enhance FallbackHandler to apply model mappings before falling back to ampcode.com
- Add structured logging for routing decisions (local provider, mapping, amp credits)
- Update config.example.yaml with amp-model-mappings documentation

internal/api/modules/amp/model_mapping.go

@@ -0,0 +1,113 @@
+// Package amp provides model mapping functionality for routing Amp CLI requests
+// to alternative models when the requested model is not available locally.
+package amp
+
+import (
+	"strings"
+	"sync"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/util"
+	log "github.com/sirupsen/logrus"
+)
+
+// ModelMapper provides model name mapping/aliasing for Amp CLI requests.
+// When an Amp request comes in for a model that isn't available locally,
+// this mapper can redirect it to an alternative model that IS available.
+type ModelMapper interface {
+	// MapModel returns the target model name if a mapping exists and the target
+	// model has available providers. Returns empty string if no mapping applies.
+	MapModel(requestedModel string) string
+
+	// UpdateMappings refreshes the mapping configuration (for hot-reload).
+	UpdateMappings(mappings []config.AmpModelMapping)
+}
+
+// DefaultModelMapper implements ModelMapper with thread-safe mapping storage.
+type DefaultModelMapper struct {
+	mu       sync.RWMutex
+	mappings map[string]string // from -> to (normalized lowercase keys)
+}
+
+// NewModelMapper creates a new model mapper with the given initial mappings.
+func NewModelMapper(mappings []config.AmpModelMapping) *DefaultModelMapper {
+	m := &DefaultModelMapper{
+		mappings: make(map[string]string),
+	}
+	m.UpdateMappings(mappings)
+	return m
+}
+
+// MapModel checks if a mapping exists for the requested model and if the
+// target model has available local providers. Returns the mapped model name
+// or empty string if no valid mapping exists.
+func (m *DefaultModelMapper) MapModel(requestedModel string) string {
+	if requestedModel == "" {
+		return ""
+	}
+
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	// Normalize the requested model for lookup
+	normalizedRequest := strings.ToLower(strings.TrimSpace(requestedModel))
+
+	// Check for direct mapping
+	targetModel, exists := m.mappings[normalizedRequest]
+	if !exists {
+		return ""
+	}
+
+	// Verify target model has available providers
+	providers := util.GetProviderName(targetModel)
+	if len(providers) == 0 {
+		log.Debugf("amp model mapping: target model %s has no available providers, skipping mapping", targetModel)
+		return ""
+	}
+
+	// Note: Detailed routing log is handled by logAmpRouting in fallback_handlers.go
+	log.Debugf("amp model mapping: resolved %s -> %s", requestedModel, targetModel)
+	return targetModel
+}
+
+// UpdateMappings refreshes the mapping configuration from config.
+// This is called during initialization and on config hot-reload.
+func (m *DefaultModelMapper) UpdateMappings(mappings []config.AmpModelMapping) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	// Clear and rebuild mappings
+	m.mappings = make(map[string]string, len(mappings))
+
+	for _, mapping := range mappings {
+		from := strings.TrimSpace(mapping.From)
+		to := strings.TrimSpace(mapping.To)
+
+		if from == "" || to == "" {
+			log.Warnf("amp model mapping: skipping invalid mapping (from=%q, to=%q)", from, to)
+			continue
+		}
+
+		// Store with normalized lowercase key for case-insensitive lookup
+		normalizedFrom := strings.ToLower(from)
+		m.mappings[normalizedFrom] = to
+
+		log.Debugf("amp model mapping registered: %s -> %s", from, to)
+	}
+
+	if len(m.mappings) > 0 {
+		log.Infof("amp model mapping: loaded %d mapping(s)", len(m.mappings))
+	}
+}
+
+// GetMappings returns a copy of current mappings (for debugging/status).
+func (m *DefaultModelMapper) GetMappings() map[string]string {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	result := make(map[string]string, len(m.mappings))
+	for k, v := range m.mappings {
+		result[k] = v
+	}
+	return result
+}