feat: add network mode for combined HTTP and SSE endpoints

Add new 'network' transport mode that serves both HTTP and SSE protocols
on the same port with different URL paths:
- HTTP endpoint: /mcp
- SSE endpoint: /sse

Changes:
- Add network mode to operation/operation.go using http.ServeMux routing
- Update cmd/cmd.go flag descriptions to include network option
- Update README.md with network mode documentation and examples
- Update config.json with network mode configuration examples

The network mode allows clients to choose between HTTP or SSE protocols
without requiring separate server instances or ports. This provides
better resource utilization and simpler deployment.

Backward compatibility maintained - all existing modes (stdio, http, sse)
work unchanged.

Based on v0.3.0 for clean upstream compatibility.

cmd/cmd.go

@@ -21,13 +21,13 @@ func init() {
 		&flagPkg.Mode,
 		"t",
 		"stdio",
-		"Transport type (stdio, sse or http)",
+		"Transport type (stdio, sse, http, or network)",
 	)
 	flag.StringVar(
 		&flagPkg.Mode,
 		"transport",
 		"stdio",
-		"Transport type (stdio, sse or http)",
+		"Transport type (stdio, sse, http, or network)",
 	)
 	flag.StringVar(
 		&host,