Config layering and profile defaults #98

Summary
Reduce CLI verbosity and allow user/global defaults.

Proposed changes

• Support a global config at ~/.config/struct/config.yaml for defaults (structures_path, input_store, file_strategy, safe hooks, etc.).
• Merge order: CLI args > project .struct.yaml > user config > built-ins.
• Expose a struct config print subcommand to show effective config.

Files

• struct_module/utils.py (read_config_file, merge_configs)
• struct_module/main.py (argument merging)
• docs/configuration.md

Acceptance criteria

• Defaults can be set via user config and overridden via CLI.
• Effective config can be printed and is documented.
• Tests for merge precedence.