fix: check config file is not world-writable before loading

[Jah-yee]

Summary

LoadConfigByPath now checks whether the config file is world-writable before reading it, using the existing but unused isFileWorldWritable function from security.go.

What changed

Added a world-writable check at the top of LoadConfigByPath in config.go (8 lines added). If the file is world-writable (or unreadable), the load is skipped and a warning is logged:

Refusing to load configuration from /etc/ussher/<user>.yml: file is world-writable

This follows the same pattern as other security gates in ussher.go (isExecutableWritable, isRunningAsRoot, isValidUser) which all prevent unsafe operation.

Why

isFileWorldWritable was defined in security.go but never called anywhere in the codebase. The config file at /etc/ussher/<user>.yml controls which URLs ussher fetches SSH keys from — a world-writable config file means an attacker could redirect key lookups.

Fixes #5

[Jah-yee]

Hi maintainer, gentle ping - this security check is ready for review. Let me know if anything needs adjustment! 🙏

[Jah-yee]

Hi @dolph 👋 Following up — this adds a world-writable permission check (8 lines) to prevent security issues when loading config. Would appreciate a review when you have time! 🙏