cli: Clarify US/HK short selling setup and note HK IRD fee

[huacnlee]

Summary

• Clarifies that US stocks can be shorted directly with no additional setup
• Clarifies that HK short selling requires activation via the mobile app: place first HK short sell order → triggers SBL agreement signing → wait for approval
• Notes that HK short selling is subject to a fee levied by the Hong Kong Inland Revenue Department, with details in the in-app agreement
• Documents API error 602301 returned before the HK SBL agreement is signed
• Updates docs/{en,zh-CN,zh-HK}/docs/cli/orders/order.md and skills/longbridge/references/cli/overview.md