Closes #326

Summary

Add comprehensive documentation for the configuration subsystem (docs/CONFIG_GUIDE.md) covering all 4 header files:

• config_watcher.h — Hot-reload file watching with platform-native APIs (inotify/kqueue/ReadDirectoryChangesW)
• unified_config.h — Type-safe configuration schema with 7 sub-structs
• cli_config_parser.h — Command-line argument parsing and configuration overrides
• config_loader.h — YAML file loading, environment variable substitution, and validation

Documentation Contents

1. Configuration Architecture

• Configuration source priority (CLI > Env > YAML > Defaults)
• Hot-reload mechanism with debouncing
• Architecture diagram

2. Unified Config

• All 7 sub-structs documented: thread, logger, monitoring, database, network
• Default values for each field
• Hot-reloadable field identification

3. Config Watcher

• Cross-platform file system monitoring
• Callback registration patterns
• Version tracking and rollback support
• Thread safety guarantees

4. CLI Config Parser

• Argument format (--config, --set, --help, --version)
• Boolean flag parsing
• Help text generation

5. Config Loader

• YAML file format support
• Error handling and validation
• Environment variable substitution (${VAR_NAME})

6. Complete Usage Examples

• Basic configuration loading
• Full-featured hot-reload example
• Hot-reload callback patterns

7. Environment Variable Reference

• All UNIFIED_* environment variables documented

8. YAML Configuration Examples

• Minimal, production, and environment-substitution examples

9. Hot-Reload Patterns

• Selective hot-reload
• Validation before apply

10. Troubleshooting

• Common errors and solutions

Test Plan

• Documentation follows existing style (INTEGRATION_GUIDE.md)
• All 7 configuration sub-structs documented
• All 4 header files covered
• Complete usage examples provided
• Environment variable reference included
• YAML examples provided
• Hot-reload patterns documented
• Troubleshooting section added

Related Files

• include/kcenon/common/config/config_watcher.h
• include/kcenon/common/config/unified_config.h
• include/kcenon/common/config/cli_config_parser.h
• include/kcenon/common/config/config_loader.h