Add multi-source configuration support

[HartBrook]

Summary

• Adds multi-source configuration support allowing users to pull languages, commands, and base configs from different GitHub repositories
• Refactors sync options into named methods for improved readability
• Extracts shared config merging logic into reusable helpers
• Adds IsNotFoundError helper for better error handling in fetch operations
• Improves error logging: distinguishes 404s from other errors, warns appropriately for explicitly configured vs default items

Test plan

• All existing tests pass
• New unit tests for multi-source config detection and resolution
• New integration tests for multi-source language configurations
• New YAML fixture tests for multi-source scenarios (basic, fallback, multiple languages)
• Verified deterministic output ordering with sorted map iteration