# Task 0.2.3: Implement Config Loader

## Metadata
- **Task ID**: 0.2.3
- **Title**: Implement Config Loader
- **Phase**: 0 - Project Setup & Foundation
- **Section**: 0.2 Configuration System
- **Status**: Pending
- **Priority**: High
- **Estimated Time**: 30 minutes
- **Dependencies**: 0.2.1, 0.2.2

## Description
Implement the Viper-based configuration loader in `internal/config/` that implements the `ConfigProvider` interface. This loader will handle hierarchical configuration loading from files and environment variables.

## Requirements
- Implement `ConfigProvider` interface using Viper
- Load configuration in order: defaults → environment-specific → env vars
- Support YAML configuration files
- Support environment variable overrides
- Provide placeholder for secret manager integration (Phase 6)

## Implementation Steps
1. Create `internal/config/config.go`:
   - Implement `ConfigProvider` interface
   - Wrap Viper instance
   - Implement all interface methods
2. Create `internal/config/loader.go`:
   - `LoadConfig()` function
   - Load `config/default.yaml` as baseline
   - Merge environment-specific YAML (development/production)
   - Apply environment variable overrides
   - Set up automatic environment variable binding
3. Add error handling for missing config files
4. Add logging for configuration loading

## Acceptance Criteria
- [ ] `internal/config/config.go` implements `ConfigProvider`
- [ ] `internal/config/loader.go` has `LoadConfig()` function
- [ ] Configuration loads from `config/default.yaml`
- [ ] Environment-specific configs are merged correctly
- [ ] Environment variables override file values
- [ ] Errors are handled gracefully

## Related ADRs
- [ADR-0004: Configuration Management](../../adr/0004-configuration-management.md)

## Implementation Notes
- Use Viper's `SetConfigName()` and `AddConfigPath()`
- Use `MergeInConfig()` for environment-specific files
- Use `AutomaticEnv()` for environment variable binding
- Set environment variable prefix (e.g., `GOPLT_`)
- Use `SetEnvKeyReplacer()` to replace dots with underscores

## Testing
```bash
# Test config loading
go test ./internal/config -v
```