feat: add prioritize-model-mappings config option

Add a configuration option to control whether model mappings take
precedence over local API keys for Amp CLI requests.

- Add PrioritizeModelMappings field to AmpCode config struct
- When false (default): Local API keys take precedence (original behavior)
- When true: Model mappings take precedence over local API keys
- Add management API endpoints GET/PUT /prioritize-model-mappings

This allows users who want mapping priority to enable it explicitly
while preserving backward compatibility.

Config example:
  ampcode:
    model-mappings:
      - from: claude-opus-4-5-20251101
        to: gemini-claude-opus-4-5-thinking
    prioritize-model-mappings: true

internal/api/modules/amp/routes.go

@@ -171,7 +171,7 @@ func (m *AmpModule) registerManagementRoutes(engine *gin.Engine, baseHandler *ha
 	geminiBridge := createGeminiBridgeHandler(geminiHandlers.GeminiHandler)
 	geminiV1Beta1Fallback := NewFallbackHandlerWithMapper(func() *httputil.ReverseProxy {
 		return m.getProxy()
-	}, m.modelMapper)
+	}, m.modelMapper, m.getPrioritizeModelMappings)
 	geminiV1Beta1Handler := geminiV1Beta1Fallback.WrapHandler(geminiBridge)
 
 	// Route POST model calls through Gemini bridge with FallbackHandler.
@@ -209,7 +209,7 @@ func (m *AmpModule) registerProviderAliases(engine *gin.Engine, baseHandler *han
 	// Also includes model mapping support for routing unavailable models to alternatives
 	fallbackHandler := NewFallbackHandlerWithMapper(func() *httputil.ReverseProxy {
 		return m.getProxy()
-	}, m.modelMapper)
+	}, m.modelMapper, m.getPrioritizeModelMappings)
 
 	// Provider-specific routes under /api/provider/:provider
 	ampProviders := engine.Group("/api/provider")