Flatpak CommandRunner Migration (Issue #20 Part 3)

## Summary
Migrate Flatpak package manager from direct `exec.Command` calls to unified CommandRunner interface for consistent, testable command execution architecture.

**This is Part 3 of Issue #20 - CommandRunner Pattern Implementation**

## Background
Complete the CommandRunner migration initiative by implementing unified command execution patterns for Flatpak, achieving full architectural consistency across all package managers.

## Current State
- **APT**: ✅ Uses CommandRunner (Issue #27 completed)
- **YUM**: ✅ Uses CommandRunner (already implemented)
- **Snap**: Direct `exec.Command` calls (Issue #28 - Part 2)
- **Flatpak**: Uses direct `exec.Command` calls

## Scope
- **Flatpak package manager migration** from `exec.Command` to CommandRunner
- **Constructor standardization** following established APT/YUM/Snap patterns
- **Dependency injection** for comprehensive testing capabilities
- **Test coverage expansion** (currently 0% coverage for Flatpak)

## Technical Implementation

### CommandRunner Integration
- Replace all `exec.Command` calls with CommandRunner interface
- Implement automatic LC_ALL=C handling for consistent English output
- Add built-in interactive mode support via `RunInteractive()`
- Enable dependency injection for testing

### Constructor Pattern
Following established standardization:
- `NewPackageManager()` - Production use with default CommandRunner
- `NewPackageManagerWithCustomRunner()` - Testing use with mock CommandRunner

### Testing Infrastructure
- Implement comprehensive mock-based testing
- Add environment variable tracking capabilities
- Create cross-platform test scenarios
- Achieve significant test coverage improvement (currently 0%)

## Benefits

### Architecture Benefits
- ✅ **Complete consistency** - All package managers use same CommandRunner pattern
- ✅ **Automatic LC_ALL=C** - No manual environment setup needed
- ✅ **Built-in interactive support** - Proper stdin/stdout/stderr handling
- ✅ **Unified codebase** - Same patterns and interfaces everywhere

### Testing Benefits
- ✅ **Consistent testing** - Same mocking approach across all package managers
- ✅ **Environment testing** - Built-in environment variable verification
- ✅ **Cross-platform testing** - Same tests work across all environments
- ✅ **Full coverage** - All command execution paths testable

### Project Completion
- ✅ **Issue #20 completion** - All package managers migrated to CommandRunner
- ✅ **Architectural consistency** - Unified command execution across entire codebase
- ✅ **Testing maturity** - Comprehensive test coverage for all package managers

## Acceptance Criteria
- [ ] Flatpak migrated from direct exec.Command to CommandRunner
- [ ] Constructor naming follows established patterns
- [ ] Dependency injection implemented for testing
- [ ] Comprehensive test coverage added (target: >80%)
- [ ] LC_ALL=C automatically handled
- [ ] Interactive mode works consistently
- [ ] All existing functionality preserved
- [ ] Documentation updated
- [ ] **Issue #20 marked as complete**

## Priority
**Medium Priority** - Final piece of architectural consistency improvement.

## Dependencies
- Should follow patterns established in Issue #27 (APT) and Issue #28 (Snap)
- Can be implemented in parallel with Issue #28 (Snap)

## Related Issues
- **Completes** Issue #20 - CommandRunner Pattern Implementation
- Part of the same initiative as Issue #27 (APT) and Issue #28 (Snap)

## Implementation Notes
- Follow proven patterns from APT migration (Issue #27)
- Leverage existing CommandRunner interface and MockCommandRunner
- Maintain backward compatibility during migration
- Use established testing frameworks and patterns
- **This completes the full CommandRunner migration across all package managers**