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/amp.go

@@ -100,6 +100,16 @@ func (m *AmpModule) Name() string {
 	return "amp-routing"
 }
 
+// getPrioritizeModelMappings returns whether model mappings should take precedence over local API keys
+func (m *AmpModule) getPrioritizeModelMappings() bool {
+	m.configMu.RLock()
+	defer m.configMu.RUnlock()
+	if m.lastConfig == nil {
+		return false
+	}
+	return m.lastConfig.PrioritizeModelMappings
+}
+
 // Register sets up Amp routes if configured.
 // This implements the RouteModuleV2 interface with Context.
 // Routes are registered only once via sync.Once for idempotent behavior.