feat: add --config flag and SL_CONFIG env var for config path

[fdematos]

Summary

• Add --config CLI flag to specify configuration file path
• Add SL_CONFIG environment variable as fallback
• Priority: flag > env > default (config.yaml)
• Rename Go module from github.com/fvigneault/smart-llama to github.com/smartfrog/smart-llama

Usage

# CLI flag (highest priority)
./smart-llama --config /path/to/config.yaml

# Environment variable (medium priority)
SL_CONFIG=/path/to/config.yaml ./smart-llama

# Default (lowest priority)
./smart-llama  # uses config.yaml

Testing

• Added unit tests for resolveConfigPath() covering all priority scenarios
• All existing tests pass with the module rename