feat(config): override config with environment variables

[roshii]

This PR adds support for overriding JoinMarket configuration settings via environment variables, enabling easier deployment in containerized environments and CI/CD pipelines.

Changes

Core Functionality

• Added override() function that reads environment variables prefixed with JM_ and applies them to the configuration
• Environment variables are automatically converted to lowercase config keys and mapped to appropriate sections
• Support for sections with subsections (e.g., JM_MESSAGING_ONION_TYPE=onion maps to [MESSAGING:onion] type=onion)

Refactoring

• Split load_program_config() into smaller, focused functions:
  • set_paths() - handles data directory and config file path setup
  • read_config_file() - reads existing config file with error handling
  • write_config_file() - creates new default config file\

Usage Examples

# Override blockchain network
export JM_BLOCKCHAIN_NETWORK=testnet

# Override policy settings
export JM_POLICY_TX_FEES=1000

# Override messaging subsection
export JM_MESSAGING_ONION_TYPE=onion

# Override daemon port
export JM_DAEMON_DAEMON_PORT=27184

Benefits

• Container-friendly: No need to modify config files in Docker containers
• CI/CD integration: Easy configuration overrides in deployment pipelines
• Security: Sensitive settings can be injected via environment variables
• Flexibility: Maintains backward compatibility with existing config files

This feature follows the common pattern of using prefixed environment variables for configuration overrides, making JoinMarket more suitable for modern deployment workflows.