Add 'silent' log level to config schema

- Support MCP_LOG_LEVEL=silent for stdio mode
- Fixes ZodError: Invalid enum value
- Allows complete logging suppression for MCP protocol

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/config.ts

@@ -7,7 +7,7 @@ const ConfigSchema = z.object({
   mcp: z.object({
     host: z.string().default('127.0.0.1'),
     port: z.number().default(19017),
-    logLevel: z.enum(['debug', 'info', 'warn', 'error']).default('info'),
+    logLevel: z.enum(['debug', 'info', 'warn', 'error', 'silent']).default('info'),
   }),
   nats: z.object({
     url: z.string().default('nats://localhost:4222'),