Add /island info command with island summary output

[ChengPeng-1010]

Summary

• add new /island info subcommand
• show island age, build range (X/Z), island level, member count, and challenge progress
• register command in IslandCommand and include in help text

Why

Issue #9 requests /is info style summary information for island status. This introduces an MVP command with the most useful core fields in one response.

Notes

• age is derived from world folder last-modified time
• build range uses world border size, with config fallback

Closes #9