Refactor file utilities into dedicated package and improve plugin validation

[peteski22]

Summary

Creates internal/files package to consolidate file and directory utilities, and adds validation to ensure configured plugins exist at config load time.

Changes

• Created internal/files with utilities extracted from internal/context:
• XDG directory helpers: UserSpecificConfigDir(), UserSpecificCacheDir()
• Permission validation: EnsureAtLeastSecureDir(), EnsureAtLeastRegularDir()
• Plugin discovery: DiscoverExecutables(), DiscoverExecutablesWithPaths()
• Helper functions: AppDirName()
• Added directory and plugin existence checks to PluginConfig.Validate():
  • Verifies plugin directory is accessible
  • Confirms all configured plugins exist as executables
  • Returns descriptive errors for missing plugins